We are using the XL2 and I'll be shooting 1 talking figure against a
green screen with professional lighting done by a studio.

I was wondering which mode of shooting is better for me to get a
cleaner key?

I came across this thought because i was looking for solutions for
keying problems for when they are quick movements - can't get a clean
image to key. Of course our general key can be better as well. I
thought about faster shutter speed as well. Shooting in 25P can look
good and was thinking - it will be great if it improves image for
chromakey

any advice?

I tried shooting in 24p, and it looked bad, but I think I lit it bad. Lighted again and shot in 60i, and it looked ok. Also, I read on this board about eliminating shadows by lighting the chromakey separately, moving your subject farther out in front of the back drop, and back light your subject with a yellow light (haven't tried that, yet). Here's a link to a tutorial with Final Cut Pro/green screen work. After hearing about Robert Rodriguez's Sin City processes, I have been fascinated with this subject. Good luck. hth.
http://www.dvformat.com/2002/09_sep/tutorials/fcpqt52.htm