I think TradeMonster might be the right answer

After spending quite a while looking at different online brokers, I think TradeMonster might be the one I’ll choose. Its commissions ($7.50/trade) are higher than TradeKing and Zecco (and barely higher than Scottrade), but I pay a lot less in commission every year than I do in margin interest. Trademonster’s margin interest rate is super-low, around 3.5% for $0-50,000. This will save me hundreds and hundreds of dollars per year over Scottrade. The cost in commissions will be a bit, but I’m estimating to save money overall. For a heavy margin trader that doesn’t day-trade, this is a pretty good deal.