I think the most used brokers by professional retail traders are;

Interactive brokers,
Tradestation
ThinkorSwim

If you only goal is to trade IWM or GDX -- probably any major or (semi)major online broker will do. IB has the advantage that you can keep your account currency in EURO. Which is a great advantage for those of you who count in Euro. TOS is USD based, and it might even not accept new European clients -- not sure what the current policy is.