Since I'm already running 0710, I'm inclined to work on that branch. However, if it's close enough to the 0810 release or if there have been enough changes to UpdateMedia since 0710 to make merging across branches impractical, I'll take a look at trunk instead.
I know what you mean, and that's the direction I initially went with some UpdateMedia changes too. It worked out fine starting out with 0710 and I added some specific steps that helped me compile UpdateMedia under 0710 (http://wiki.linuxmce.org/index.php/Building_From_Source#Building_UpdateMedia
), but keep in mind that (a) the 0810 build is a little more sane to work with (IMO anyway) and (b) patches won't be accepted against the 0710 branch. However, it was an easy merge when I ultimately made the switch to 0810, so I think you will be fine.
One last note - if you decide to go for 0810 for development, you want the 0810 branch, not trunk! That was a mistake I made as well
. Some great instructions here for setting up a chroot dev environment: http://wiki.linuxmce.org/index.php/Building_LinuxMCE_0810