I assume you are invested in Canon lenses, therefore switching to a GH4, A7s, or Sony NEX would be way more expensive than your target budget (SL1 range). Yes the best video cameras on the market are the A7s and GH4, but these are in the 5D price range not the SL1, especially since you'll re-invest in a new lens system. If you have the budget and serious about filmmaking, certainly go for it.
However back to the original question, a light cheap video camera to use on a steadicam to match the 5D II,
I suggest you look into the Canon EOS M. The camera may not be popular with photographers due to the slow AF, but it's quite good for video. If you don't need the AF and Optical VF of the SL1, the eos m gives you the exact same video quality (which matches the 5D II very well) in a smaller, and far cheaper package. The price is ridicuously low now. It's going for around 200-300 dollars new. And you can use your existing Canon lenses of course. The perfect camera for steadycam work.
The Eos m will give similar video quality to the 5D II therefore if you're fine with the 5D you'll be pleased with it.
But if you're bothered by the moire and aliasing on the 5D II (& eos m) and want a slightly sharper image than the 5D, take a look at Sony A5100 and A6000. Both go for around 600$. But again another lens system and nore expenses, but yes better image quality - no aliasing and slightly more detailed.
If you find the 5D mk II image quality adequate for your needs, I think pairing it with a small eos m as a B cam is the best option. Both make very powerfull video tools.
PS- tip: install Magic Lantern on both cameras. It's a must for video shooters. It takes these cameras to a whole new level adding professional video tools like focus peaking, zebras, waveform monitor, audio meters, intervalometer, and tens of other priceless features. It's extremely safe and stable that I trust using it on paid work.
Another tip: in the picture style controls, choose Neutral and turn the contrast and sharpness all the way down. This is the first thing to do before using a Canon DSLR for video, it gives a much more pleasing image with higher dynamic range. Not doing this and leaving it to Canon's standard settings gives a horriblybover-sharpened and over-contrasty digital-looking image.
Tks Ebrahim Saadawi for recommending EOS M! Your suggestions are vy enlightening & relevant!
Thanks for all the good tips!
Indeed, all I have are EF lenses... and I am thinking about using a 18-55 set to 20mm on the SL1 and
use fixed manual focus only. On steadycam use outdoors, an aperature of f5.6 should be sufficient to keep
the subject sharp, if I keep the shooting distance roughly the same.
The fact that EOS M video quality can match up to 5D2 is good to know. What is the highest ISO have you shot on EOS M to match 5D2 quality?
I think I will need to get the EOS M to EF adaptor too... Also need to check how mature Magic Lantern is for EOS M. Might be a bit further on SL1 too... not sure...