Tradestation has a bunch of charting and system design and testing capabilities IB doesn't have. Study the websites of both in detail and decide if, for you, the Tradestation features are worth the additional cost.
I'm not so sure about that short availability. I haven't traded with my Tradestation yet, but they have a boatload of stocks marked HB(Hard to borrow). IB rarely has a problem with shorts.
If The Bear can not borrow it, It is not borrowable.
I have a Stock Loan Buddy that tells me if an issue is borrowable.
I have never had any problems with TS shorting.