i wanna use an l293d for differential drive.......i have seen two power supply to it one 5v and other 12v.....is it advisable to connect it so.....also for the 5v can i use the same supply as for the uc
I would be careful here. The motors can create voltage spikes that could go all the way back to the uC. I would have the motors on their own supply, or a non regulated voltage. If you want the motors to run on 5V, have them run on their own 5v regulator.
Capacitors across the motors and power supply will also help.
Either is, essentially, identical to the other. Minor differences in specs (more functions in the L293D, but it's pricier, too), and from what I could tell with a quick read-up of the BA6218, differences in package types.