Even i am not sure about it. May be they are just model numbers. The main difference between the two models is 2490BT - touchscreen unit and 2490UB - single DIN player/receiver.
As for 2490UB it seems to be decent for general use. I haven't used it personally. If you are planning to add a woofer in future i would suggest you to go for a player with 3 RCA outputs just by adding another 1-1.5K.