Depends on when you need to show your cert. If you need it right away, 1.4. If you can wait, and you know 1.5 features, 1.5. In job interviews on both sides (interviewer/interviewee), I've never had anybody ask what version a Sun cert was. Never. I've heard questions about the significance of each cert, but never the version...
Give a man a fish, he'll eat for one day. Teach a man to fish, he'll drink all your beer.
Cheers, Jeff (SCJP 1.4, SCJD in progress, if you can call that progress...)