If you want to trade forex AND futures AND equities, then the choice is IB since all funds are kept in the same universal account and moved around automatically, as needed, for your various trades. With MBT, YOU have to move funds between your SIPC insured account into your forex account as needed if you want to trade all three.
As to cost, a commission comparison shows that IB is the lower cost of the two (MBT does have a equity all-in commission, but as has been pointed out on these boards before, you may not be getting the best fills with that option).
Customer service-wise - well IB really doesn't have any, so if you need that feature, MBT wins.
Spreads: spreads can be tighter on MBT. IB keeps their spreads quoted in 1/2 pip increments.