I've shot greenscreen heavy pieces with the GH13 (4:2:2 MJPEG), Gh2 (hacked), XF300 and the HPX170. IMHO, pulling from 4:2:2 is always more fun than 4:2:0. You can get "acceptable" results with high-res 4:2:0, though it all depends on what you find acceptable. Pulling the green out on the GH2 is about 80% acceptable for me, but fringe cases (poor lighting, hair, translucency, bad spill, black/white cg backdrops, etc.) can result in many, many more hours of post work to get it to look "perfect". This may or may not be a big deal to you, but depending on your final delivery format, client and deadline, this can make or break a shot. I had very tight deadlines on several green screen heavy projects and being able to shoot 4:2:2 helped greatly. Given that my compositing post-workflow can be quite long, any chance to reduce the time spent on any component of that workflow is very important for me. The question of 4:2:2 to 4:2:0 to me is like HDV tape vs. CF/SDHC digital workflows. Both are capable, the latter is faster and more fun.
Even though the HPX170 suffers from lower native resolution, pulling keys are much less painstaking than vs. the 4:2:0 and edges are much less aliased due to better chroma. I shot a side-by-side comparison between the 5Dii, GH2 and HPX170 on a decently lit green screen, and while it killed me to give up the resolution of the 5D and GH2, I found keying the HPX170 gave me much cleaner edges and better organic results in much less time, so I went with the HPX170 for that project. The EX3 performs better than the HPX170 even though it's 4:2:0 because of its 1/2" 1920x1080 chips vs the lower native res of the HPX. With that in mind, the Canon XF300 with the combo of full raster 1920x1080 3MOS and 4:2:2, has held up very, very well for green screen work. I haven't tested the XF100, but I imagine it to be very similar.
In ideally lit scenarios, the differences between the cameras will probably be un-noticeable except to pixel peepers. In real life though, outside of the studio, time constraints may often have you creating green screen shots with less than ideal (wrinkled) green screens and very un-even lighting. In these cases, you'll be much happier with a camera/system capable of 4:2:2.
I'm not sure if you're asking for this, but if you need a camera capable of shallow DOF AND good green screen capabilities (aka, you can only afford one camera), then you might want to consider the AF-100 or the FS-100 with the Ninja. If you can only afford the DSLR range, then the hacked GH2 would be your best bet. I personally own multiple cameras specifically for compositing reasons and prefer shooting green screen with small chip camcorders for their 4:2:2 and deep DOF with decent low-light performance. If I need shallow DOF for most scenes, I usually apply camera blur in post, but this may vary depending on the types of scenes you're shooting.
You might have already made up your mind, but here is an uncomprehensive list of 4:2:2 capable cameras/combos (older and newer) that you might consider:
Panasonic HVX200a (used) (960x540, SDHC, poor low-light): ~$2k
Panasonic HMC150 (used) (960x540, SDHC) + ATOMOS ninja: $3k
Panasonic HPX170 (used) (960x540 native res, P2 cards): ~$3-4k?
Panasonic AG160 + Ninja (3MOS, 1920x1080)= $4.5 + $1k = $5.5k
Canon XA 10 (no 720p/60?) + Ninja = $2k + $1k = $3k
Canon XF100 (CMOS, 1920x1080, CF cards)= $3k
Canon XF300 (3MOS, 1920x1080, CF cards) = $6.5k
I left out Sony/JVC as their price points are currently a bit higher.
Like others have stated, I would recommend the XF100 as a solid budget 4:2:2 camcorder. As a bonus, color grading 4:2:2 is much more fun as well
Even though the HPX170 suffers from lower native resolution, pulling keys are much less painstaking than vs. the 4:2:0 and edges are much less aliased due to better chroma. I shot a side-by-side comparison between the 5Dii, GH2 and HPX170 on a decently lit green screen, and while it killed me to give up the resolution of the 5D and GH2, I found keying the HPX170 gave me much cleaner edges and better organic results in much less time, so I went with the HPX170 for that project. The EX3 performs better than the HPX170 even though it's 4:2:0 because of its 1/2" 1920x1080 chips vs the lower native res of the HPX. With that in mind, the Canon XF300 with the combo of full raster 1920x1080 3MOS and 4:2:2, has held up very, very well for green screen work. I haven't tested the XF100, but I imagine it to be very similar.
In ideally lit scenarios, the differences between the cameras will probably be un-noticeable except to pixel peepers. In real life though, outside of the studio, time constraints may often have you creating green screen shots with less than ideal (wrinkled) green screens and very un-even lighting. In these cases, you'll be much happier with a camera/system capable of 4:2:2.
I'm not sure if you're asking for this, but if you need a camera capable of shallow DOF AND good green screen capabilities (aka, you can only afford one camera), then you might want to consider the AF-100 or the FS-100 with the Ninja. If you can only afford the DSLR range, then the hacked GH2 would be your best bet. I personally own multiple cameras specifically for compositing reasons and prefer shooting green screen with small chip camcorders for their 4:2:2 and deep DOF with decent low-light performance. If I need shallow DOF for most scenes, I usually apply camera blur in post, but this may vary depending on the types of scenes you're shooting.
You might have already made up your mind, but here is an uncomprehensive list of 4:2:2 capable cameras/combos (older and newer) that you might consider:
Panasonic HVX200a (used) (960x540, SDHC, poor low-light): ~$2k
Panasonic HMC150 (used) (960x540, SDHC) + ATOMOS ninja: $3k
Panasonic HPX170 (used) (960x540 native res, P2 cards): ~$3-4k?
Panasonic AG160 + Ninja (3MOS, 1920x1080)= $4.5 + $1k = $5.5k
Canon XA 10 (no 720p/60?) + Ninja = $2k + $1k = $3k
Canon XF100 (CMOS, 1920x1080, CF cards)= $3k
Canon XF300 (3MOS, 1920x1080, CF cards) = $6.5k
I left out Sony/JVC as their price points are currently a bit higher.
Like others have stated, I would recommend the XF100 as a solid budget 4:2:2 camcorder. As a bonus, color grading 4:2:2 is much more fun as well