i am not whole without my glock... the xd and mp are good choices but the m&p hasnt been out long enough to prove itself while you cannot buy parts if they break on either, so a week or longer of downtime (which i hate) the m&p mags are pretty expensive xd is half the price, on the ease of taking down both are the exact same, lever in the front, i think an xd would be a better choice but i would still put glock as #1 for me, trust me before i got the glock i was in the same boat your in, and as for changing calibers xd has 40 and 357 but the glock has 9mm (conversion bbl) 40, and 357 and the mags are cheap and parts out the @$$ and you can get .22 conversions, value: if you do sell it xd and glock would probably be around the same price
heres a quick list if you dont want to read
XD:
Pros: cheap, reliable, accurate, 2 caliber conversions available
tough finish, and great warranty
Cons: no parts available, no aftermarket mods (as of now), the mags are 2-5.00 more than glock mags (if u go to the right place) not the easiest to work on (my friend made me take a crack at his)
Glock:
Pros: its a glock
jk, carried by many many cops, utterly reliable, parts every where, 4 different caliber conversions, cheaper mags, lots of custom parts, great people at glock they will fix anything wrong, and a good finish
Cons: higher chance of a kB, and factory plastic sights seem to fall off after a while.