Yay! more data! Hmm your view on Celestron greatly differs from those at the local astronomy club . They tend to hold the Advanced VX and CGEM mounts to a pretty high standard. I think it's sticker shock, photographers have already come to terms with $1k tripods and $2k+ lenses. The astronomers in the club haven't quite crossed that bridge yet .
Celestron equipment has a very strong religious following. I was hooked on getting a CGEM DX until I started researching, asking questions. Turns out that the CGEM and CGEM DX both have a flawed gearbox that causes what is called the 8/3 error, a non-periodic, non-integer error that is extremely difficult to guide out. I spent over two solid months researching mounts. In the grand scheme of things, my "real" mount will ultimately be the 10Micron GM2000HPS UP, a $24,000 mount which uses in-mount sky modeling and absolute encoding to allow for 20 minutes unguided exposures, and can hold up to 132lb of instrument capacity.
In my travels around internet forums during my research, though, I found one very glaring fact: Pretty much NO ONE, EVER, complains about the Orion Atlas or EQ6 mounts. They have about as pristine a reputation as I have ever seen. They are very well loved mounts. This is in great contrast to the fact that you can find dozens, if not hundreds, of complaint threads about the CGEM mounts on astronomy forums all over the net. Mostly about gearbox issues, but not solely. Some Celestron fans and objective mount reviewers will tell you the complaints are not warranted, and perhaps not...but that does not change the fact that the CGEM mounts are widely complained about buy a LOT of people. Statistically, that has to indicate some fundamental issue.
Personally, I LOVE Celestron OTAs. Their EdgeHD scopes are amazing, although they do suffer a bit from the standard SCT problems. If you want an excellent large-aperture OTA, an EdgeHD is definitely a worth while investment. Just...put it on an Atlas, instead of a CGEM.
The scope according to the website is about 12lb (I threw it on a scale and it's about right). The 23lb you see includes the metal case it comes with. I know it doesn't change the P2P error, but in terms of load, 120ED + 5D should be ok on the AVX?
If the weight is only 12lb, then it should be OK. I'm not sure what imager your using, if it's a DSLR that could add another pound or so. The guiding setup will add another pound or so, plus don't forget to count the weight of the various cables that you'll need to control everything (cable from the camera to laptop, cable from the guidecam to the mount, cable from the guidecam to laptop, cable from mount to laptop). You might also have additional weight from an extra vixen dovetail and telescope rings (to mount the guidescope to the telescope), which also adds a couple of pounds. Even assuming the scope is 12lb, 50% capacity is only 15lb, and all these accessories are going to put you over that limit.
The AVX is generally not considered a great mount for doing astrophotography. It's great for visual observing, but you have to understand the tolerances involved in astrophotography...if your tracking is off by arcSECONDS, your going to have problems. The AVX is the rock-bottom mount you could possibly get for AP, and it really is insufficient. That assumes that you never, ever plan to use a larger scope in the future...if you do, the $1000 on the AVX is just a waste, as you'll need a larger mount for a larger scope in the future anyway. (You will also quickly find that you'll want a longer scope, much longer, for doing anything other than nebula wide field shots...so something like the 8" EdgeHD or 8" AT8RC, both very cost effective OTAs that produce superb results, would work on an Atlas, they definitely would not work on an AVX.) You would be surprised how much better the Atlas/EQ6 is. Some astrophotographers have loaded it up with 30, 35 pounds of weight and been able to image fine...you would never be able to do that with an AVX.
The price of the Atlas is $1500, so it's $700 more than the AVX. I know that sounds like a lot...but I honestly cannot stress enough how important the mount is for astrophotography. The difference between what is acceptable for visual observing (which is probably what most of your local astronomy club members are doing), and what is acceptable for astrophotography is quite large. You can deal with stars and nebula and planets bouncing around a bit for visual work...even the smallest amount of that is completely unacceptable for astrophotography. If you don't eventually plan on getting a really large OTA that weighs over 50lb, then the Orion Atlas or SkyWatcher EQ6 would probably be the only mount you would ever need...buy it once, never need to replace it or buy a bigger one. The same is not true of the AVX. Your already pushing it's capabilities with your ProED 120.
I guess the cost difference of 900 vs 1500 is peanuts compared to everything else >.<
It really is. The mount is the centerpiece. If your mount isn't up to snuff, then it really doesn't matter what your mounting onto it...your already screwed.
Big thing to keep in mind, visual is very different, in terms of requirements and what's acceptable, from astrophotography. Local astronomy clubs tend to be based on visual observing, and less on astrophotography, so their advice is likely to be a bit biased.