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I always thought XPress was pretty much WordPress with a little messy integration into XOOPS.
Switching WordPress would gain you XOOPS, but would limit you in many other ways, for example imagine there is a security risk in WordPress and they fix it, now XPress is also use the old version and has that security risk but you can't do anything with XPress except hope that security risk is not taken advantage off until the developer updates it to the latest WordPress Version also.
With regards to importing, I should imagine the tables would pretty much be the same, as XPress is still wordpress, although I'm not sure what version of wordpress Xpress useses and what database differences there is from your version number and intergenerated version number.
I hope that makes some sense, lol
Personally I wouldn't bother with XPress, I'd consider something like the Life Type module, I've heard from other established developers working around XOOPS that this was coded better.
If you don't really need a site for anything else other than bloggin, then WordPress is fantastic as a stand alone product, I use both WP and WPMU and love them both.