mbradley,
I've traded both the TWS and JTrader. About 2500 trades on TWS and only around 300 on JTrader so far (it's the platform that I'm currently using). I think they are both excellent programs, and I really have nothing negative to say about either.
The differences will depend on your personal tastes and trading style. I've found that JTrader is highly customizable, which I really like. TWS is somewhat less so. Both systems are very fast, but in the case of JTrader, I've been able to customize it to send my order to Globex2 in one click. ****I would caution anyone to be extremely careful before doing this.**** It is a very powerful feature which can be a really good thing or a really bad thing. I like the portfolio viewing features in JTrader. It'll show you your tick by tick P/L by symbol. (I'm an ES and NQ trader, so mine's pretty much a two liner.) In TWS, you'll see your account balance in total tick by tick--again, just a matter of preference.
PatSystems is the software company that makes and upgrades JTrader. Many brokers use the software and pay a license fee, so it's popular and well supported. I know of brokers that will charge their customers for the use of the platform. Mine does not, but I'm doing a fair volume of trades--for a small fish trader, that is.... Each Introducing Broker will have different requirements, so go through the details with any you are interested in.
The technology and speed of both are comparable--quite fast. Trading the Globex on either of these platforms is like trading the ISLD ECN. If you hit the bid or ask with a limit order, you are filled within a second.
I hope this answers your question. You won't go wrong with either platform. Your choice will probably be better directed based on the brokerage relationship that you prefer: commission structure, daytrading margin, support, etc.