I have been trade with TW about 6months and have smaller account than my IB acct. For execution quality, sometimes TW has better execution than IB. It happened with my last 2 cycles for PCLN calendar. With identical limit price on both IB and TW, TW got filled but IB didn't. My main concern about TW, it hasn't OCO/OCA function yet that very helpful to me for placing calendar spread. I often enter many calendar order with different strikes for high price stock, such as PCLN, GOOG, etc. So, i can conclude that execution quality between IB and TW is not much different.
Capped commission on TW also give some edge if we want to play with cheap calendar or more than 10 contracts on single leg. This cycle, i played with SWKS 1-week put calendar for around .20 Debit for each spread. Let's say with 10K portfolio, it need 25 spreads for half allocation ($500). With IB, it takes around $65-$70 comm for round-turn. With TW, it "only" takes around $25 round turn due to free comm to close.
So, my conclusion that TW still has much room for improvement on their platform and has bright future if the comm structure doesn't change. But, i will stick my large acct with IB due to company stability.