This was discussed a few weeks ago. Generally, as I understand it it, 32
can do everything 64 can (until someone really starts to exploit 64 bit
processing), except address more than 4GB; and the 32 bit releases are
better tested, better supported and generally more capable.

Plus when I looked at Kubuntu 32-bit on live CD it solved a couple of
outstanding problems that none of the others I tried did.