As a matter of preference, I would install BT first, because I know that the Ubuntu installer (Ubiquity as well, just isn't modified as heavily as BT's seems to be) will check for other installed OSes. Maybe the BT Ubiquity does, I'm not certain.
As for disk space, I'd give the most to Ubuntu, considering that it's a desktop-focused distro, vice BackTrack as a security-focused distro. 30 gigs isn't a lot to work with, and when fully installed, BackTrack (at least mine) weighs in at ~8.45Gb, and that's not including saved files (like monster wordlists and pcap files). Consider using external storage if you can't upgrade your hdd.