The focus sensor isn't smaller but the light allowed through to the mirror to the AF sensor is proportional by the diminished light.... Canon isn't going to waste money putting a full frame size mirror into a crop body when it doesn't need it. Plus by doing so, you would see image you wouldn't be getting anyways... When you read reviews about the 5D's viewfinder, you read adjectives such as Big, bright, pictureframe... when you do the math, it makes perfect sense...
Sorry, but that's just not correct. Yes, the reflex mirror is larger in FF camera than in an APS-C camera, which menas a bigger, brighter viewfinder, but that's completely irrelevant for the AF system. First off, the reflex mirror reflects most of the light up to the viewfinder, and allows a small amount of light to pass straight through the mirror to be reflected downward by the secondary mirror. Importantly, the secondary mirror is much smaller than the main mirror, and only a small portion in the center of the main mirror is semitransparent, to allow light through for the AF system.
The key point is that for a lens with a given max aperture, the light per unit area is the same, regardless of image sensor size. The light hitting any one point on the sensor (image sensor or AF sensor) is the same. The FF sensor gathers more light because the total area of the sensor is larger, and likewise the VF is brighter because the total area of the VF is bigger. I don't know the exact measurements in real units for the dimensions of a single AF point, but for the 5DII's AF points to receive more light, the individual points would need to be larger than their 7D counterparts. I can almost guarantee that's not the case for the center AF point, since the f/2.8-sensitive 'X'-shaped (diagonal) cross sensor on the 7D sits outside the standard '+' shaped f/5.6-sensitive center AF point, meaning it is almost certainly physically larger than the 5DII's center AF point, and with a larger area and the same light per unit area, that the 7D's center AF point is getting more light than the 5DII's center AF point.
When I bounce a laser pointer off a mirror onto a wall, the brightness of the laser dot on the wall doesn't change depending on the size of the mirror I bounce it off of.
Exactly - light per unit area is the same.
And I wont argue this matter any futher. any single point in a F5.6 exposure (with the same shutter) would be letting in 2x the light in every part of the frame than a F8 exposure... Same with the mirror.
Well, I'll argue further.
The above statement is correct, but irrelevant. An f/5.6 exposure lets in 2x the total light than f/8 over the entire image circle, true. But that's true whether there's a FF sensor sitting in that image circle, or an APS-C sensor sitting in that image circle, or the even smaller AF sensor sitting in that image circle. The smaller the sensor, the less of that image circle is sampled by the sensor. Same light coming through the lens, less light detected by a smaller sensor.
You seem to be implying that the sensor size affects exposure - it does not. If you meter a scene with a given aperture, you should get the same shutter speed for both APS-C and FF (and for MF and a tiny digicam, too). Sensor size does not affect exposure, because exposure is determined by the light per unit area, which is determined by the f/number and independent of sensor size.
Hopefully, the above clearly demonstrates that the amount of light hitting both the 5DII and 7D AF points is the same. Fine, but that's not the only factor. Different AF sensors and different points within those sensors can have different absolute sensitivities. Even two AF points with the same rated sensitivity (e.g. f/5.6-sensitive horizontal lines) can have different absolute sensitivities in terms of the amount of light required to achieve a focus lock. So, even if the same amount of light is hitting the AF point, one AF point may simply be more sensitive (e.g. applies a higher internal gain while maintaining adequate S/N to achieve a lock, much in the same way that newer image sensors can achieve higher ISO values with equivalent noise).
But enough theory. How about the real world?
Overall, when comparing the 5DII AF with the 7D AF, I would rank them as follows: 5DII center AF point > 7D center AF point > 7D off-center AF points >> 5DII off-center AF points. More specifically, the 5DII's center AF point outperfoms the 7D's AF points slightly in terms of accuracy and noticeably in terms low-light sensitivity. The 7D's center AF point is quite good, and the off-center AF points of the 7D are nearly as good as the center AF point. Comparitively, the off-center AF points of the 5DII suck, and I only use them in bright light, preferably with contrasty subjects. For AI Servo tracking, the 7D wins, hands-down.
...for focus just about always use spot towards the center of the frame and recompose.
That is fine sometimes, but if you start using fast lenses (f/2 and faster) shot wide open, that technique can lead to your subject being out of focus. See this linked article
for more details on the problem with focus-recompose.
Anyway I guess my question in general is from anyone that's used both would spot AF be noticeably different on either body?
I don't think anyone has specifically addressed this issue. If by 'spot AF' you mean that mode on the 7D, there is no such mode on the 5DII. With Spot AF, the AF system reduces the effective size of the AF points. Most people aren't aware that the actual AF point is significantly larger than the little box that represents the AF point in the viewfinder. That's one thing that leads to complaints about AF - you place the box right on the small feature you want to focus on, but the actual AF point is larger, and if there's a high-contrast feature that's just outside of your selected AF point 'box' the camera will lock onto that instead of what you put the box over. The 7D's Spot AF restricts the region of the AF point used to an area approximately equal to the size of the box in the viewfinder (note: not
the tiny inner box that indicates Spot AF, but rather the larger, main AF point box).
All my lenses are f/2.8 or faster, but sometimes I do take some photos in low-light conditions without AF assist where the 7D does hunt a while.
Back to my statement above. The center AF point of the 5DII does better in low light than the center AF point of the 7D. With the center point, my 5DII will get an AF lock in lighting conditions so dim that the 7D would hunt and give up. With a Speedlite mounted, the AF assist comes on sooner and more often with the 7D than with the 5DII.
I presume you avoid the AF assist on the 7D because the strobing of the popup flash is annoying - I know that's the case for me. You might consider using a Speedlite - you can set it so the flash doesn't fire, only the AF assist lamp does, and the AF assist is a much less offensive red grid (just be sure to get one with the dedicated lamp, like the 430EX II - lower flashes, e.g. 270EX II, just strobe the main tube like your popup flash).
Hope that helps...