Not sure if anyone mentioned it yet, I didn't see it whilst skimming... you asked about what the "B" means: It is I believe mainly for 'Belt' -drive- and also that configuration of the cogs looks like a 'B'. The original Halo didn't use a belt, and some retrospectively call that one that 'A' I think.
A lot of people complain about the sound activated models (which are Reloader B's), but this is probably due to the first gen boards not being very sensitive. The 'B2' boards have greater sensitivity, and I have never ever had a problem on SP-1, TM7 (pretty quiet) or an EGO 10 I recently used. Worked flawlessly everytime.
HALO = Uses 'eyes' to detect when there is a gap in the balls so knows when to feed. (HALO B then HALO B2 then HALO V35)
Reloader = Uses Sound to hear the gun firing and feeds another ball. (Reloader B, Reloader B2)
The 'Magna' is also based heavily on the Halo and Reloader designs, and uses sound technology and a magnetic clutch system to avoid breaking balls in the hopper from too much pressure. It also can use RF signals to be activated which gains about an extra 2 balls per second more over sound or eyes in actual ball flow (rather than just claimed).
After that came the Prophecy, which I don't know much about other than it was tooless and used a different drive train and moved away from the square circuit boards... and bizarrely seems to leave the board exposed to the elements via the buttons/switches. It is a nice design and a good hopper aside from that
The bottomline line is, pretty much anything based on the HALO/Reloader platform are good second hand reliable hoppers that go for reasonable prices, which is a large part of why they are popular. There are better hoppers/loaders (like the Dye Rotor) but they command a MUCH greater premium on the 2nd hand market for something that you are never really allowed to use at the speed they can feed at anyway. What they do give you is a very cool look, low profile etc and good battery efficiency compared to older designs like the HALO. Some say the -Rotors- are more robust as well.