Have tried ICM, can recommend the same, but commissions are a bit high if you are considering it for scalping. Can't say about FP or GlobalP, though have heard a lot of people recommending them now a days. If you are open to any other option then, Tickmill or fxview can also be an good option, since commissions and spreads both are low.
I think TDAmeritrade is a great option for trading on Mac.
Low spreads is a kind of a tricky thing. Looking at some random point of time at broker spreads and comparing them is useless since chances are high that at another point of time the picture can be totally different. Instead we have to compare average spreads which is not easy since you have to gather data about spreads for some time calculated average and then compare. You have to make special parsing solution for that because broker do not give access to their historical spreads.
Have you checked out Zenfinex? Their spreads seem very competitive and they’re doing a promotion where you can get raw spreads and 0 commissions trading for 30 days / first 100 lots.
