I think you'll find all the questions relating to this contract answered by the M6E/E7 threads (of which there are several), except that the spread on M6B is usually a little wider (~4 ticks). All other characteristics are pretty much the same.
M6A is also a good one, if you're interested in the Aussie.
The other currencies (Canadian, Japanese, & Swiss) can get a little more tricky, and are significantly less liquid than the M6E, M6B, & M6A.
While we're on the topic, if you want to trade the other currencies (Micro-Canadian, -Japanese, or -Swiss), make sure you select MCD, MJY, & MSF respectively (NOT the M6C, M6J, & M6S). Reason: MCD, MJY, and MSF are settled in US $$, and will not trigger a currency-conversion, which will usually cost a conversion fee and some headache.
M6C, M6J, & M6S are all settled in the opposing currency.
The following 3 users say Thank You to ddouglas for this post: