Personally, I find sourceforge.net better in most regards. The notable exceptions are that sf.net is famous for having bugs (and sometimes long downtime - weeks) with its CVS system (but then again I detest CVS so I’m not too bothered), and that your exposure is to “all people looking for free software and libraries” rather than “people looking for gaming softare and libs on this one particular Sun site”.
Perhaps in 10 years time the number of people browsing this Sun site will be very large, but since its small, not properly linked even from java.sun.com, and relatively unknown compared to sf.net you’ll probably find you get considerably less exposure from the site itself.
Of course, if this doesn’t matter to you then you might as well go with java.net.
I strongly advise making it 1.4.x compatible. Check out Retroweaver (co-incidentally an SF project: sf.net/projects/retroweaver I believe) to get this to work. There are an awful lot of us who won’t be upgrading to 5 any time soon, partly because there are almost no non-syntax advantages, partly because it’s extra hassle, partly because of fears over untested code … but mostly because there aren’t that many people using it and Sun has artificially made it non-backwards-executable, and we want people to be able to run our java apps; we don’t all buy-in to Sun’s Microsoft-esque approach of “force everyone to upgrade even when the advantages are minimal”.