A123s can be tricky to use as they just come in one cell type. With lipos you can just pick a voltage and then decide what size lipos to get from there. A123s is the other way around. Example, my new little toy that came today:
Its an MGM 9032. I wanted to keep my A123s, but needed a reasonable cell config to do it. I'm switching my 5S2Ps to a 10S1P setup, and am looking at getting a 1515/2Y (1100kv) to go with it. I could have kept it at 5s2p and gotten a 1y with a 16018, but I wanted to go HV and super eff and cool. Any more than 10 cells it starts getting heavy and hard to fit. This controller will also allow me to run upto 2 xtra cells for some insane action @ 40V, but keeping amps below 60A hopefully. Its "only" a 1700W system, but should be more powerful than a 2000W+ XL system due to the eff increases.
A 6S lipo equiv is hard to do, as that is 14 cells A123, so really the next node seems to be 10S so you don't have to have a paralleled cell.