Q runs on Verizon's CDMA network and the BJ runs on Cingular's GSM network. Not much else that I can tell, but I am sure there are some other differences. Guess it really depends on the network you are on or moving to.
The blackjack is narrower across the front, and a little more phone like when talking with he phone against your head, but that is an opinion. Just a little nicer form for the BJ.
Sprint is CDMA, like Verizon. They both use EV-DO, which is currently in more places than the Cingular 3G network and runs at close to broadband speeds. Both Sprint and Verizon would probably run about the same speed, but I am not an expert.
The Q actually has a faster processor, with a 300mhz one versus the 220mhz one in the Blackjack.
Now, I'll have to leave it to those who have used the two devices extensively to say which one feels faster. WM5 devices have been more about how well the software has been tweaked than processor speed as of late.
one other is that the Q uses standard mini-usb while the samsung uses it's proprietary connector. The Q uses mini SD, while the Blackjack uses Micro. The samsung, however, is smaller
i heard about the Q's keyboard not being very responsive from a friend of mine who owns one...but that might've just been for that one specific one. no idea though, i've never owned or tested out either. =)