Could someone comment on which is the better choice, HVX vs. XL H1 considering the vast magority of the work I do is in SD and frequently involves chroma-keying but I still would like a migration path to HD. My understanding is the ability to record SD in DV50 with 4:2:2 color space would offer a vast improvement over the H1 recorded to standard DV.

More color space and less compression is always good for keying, so yes, the DVCPro 50 option of the HVX seems to be better for the job.

My experience is was the DVC Pro50 actually hade better contrast and less artifacts than my Sony Z1u in HDV mode which ultimately reproduced the best key

I've been posting some comparison photos between DV and DV50 on the HVX. Won't directly answer your question about XLH1 vs. HVX, but it should illustrate just how much better DV50 looks rather than DV, especially for keying.

http://www.dvinfo.net/conf/showthread.php?t=59484