steve,
since you asked... I can point to the head of a derivatives trading desk at a leading i-bank in HK who uses TWS for his PA. He pushes large volume through his platform (one of the two you mentioned above) each day on his desk. By night he trades through the TWS for his PA. He has said on numerous times that the TWS blows the other platform away. since you claim to know the universe of all insti traders, I just point to one example of someone who thinks differently.
Note, I'm not claiming that the TWS is better than TT or GL nor am I saying that they are better than the TWS. The person mentioned above has a great deal of experience and it turns out the TWS fits what he was looking for.
The fact is, many of the large institutional trading firms, will have a direct CTCI connection utilizing their own front end. Nevertheless, I'll repeat. Most large institutional desks don't care about the bells and whistles. They know what they want to do, when they want to do it and just want to place orders quickly and accurately.
Finally, most people on this board are not institutional traders. You shouldn't compare yourself to them. Most on this board are scalping. some institutions surely scalp. most however are using strategies such as arbitrage, hedging and speculation. A completely different style of trading. Thus I'd argue that stating that the big institutions use us or them isn't even much of a selling point.