Depends if your buying new or used.
If your buying new:
Halo B - Cheaper, no rip drive. (bad when your batteries die in the heat of it.) IR sensor used to feed (unreliable if you have a break in the ball stack)
Reloader B2 - Own 2 (were originally reloader B's but had the software upgraded cos the motors were dead) Instant on (don't have to find a quiet spot), Rip drive as standard, 6 speed settings, 6 microphone sensitivity settings, anti-jam software. to be honest you can't notice any difference with the 6 speeds against the original 3, and the mic settings don't seem to alter anything, set to one and have had no problem with any gun i have put them on,
programming is a bit of chew compared to the original reloader B but once you get the hang of it its ok.
If your buying new then i would say save up a bit more and get a pulse. (although i am biased as the pulse is the only hopper i have that i haven't had issues with)