OptionsXpress Better graphics and user friendly interface. I mainly swing trade, so speed isn't that important to me.
Mmm, a reason to choose a broker, though? But I think ToS's green/gray on black platform is prettier anyways - reminds me of old DOS days. I vote ToS - great execution, good spread fills, very flexible on commissions, too. Forex: At a seminar in NY last month Don Kaufman said they'd be introducing forex sometime soon.
Currently they offer limited options. The indices (ES,ER2,YM) Mini Gold & Silver and 3 grain contracts. Forex and additional futures are in the works.
Do a search for "Cox Ross binomial calculator", there are at least 5 of them for free. I like the one at snowg*.com None of them will tell you the actual margin, but you can use the OCC calculator then use one of these binomial ones to deduce how your margin would change given a change in the IV or time decay.
Small correction/clarification, TOS does NOT offer options on futures yet, just the futures outlined above and E-mini EuroFX.