I have been using both for many years.
They are both good reasonably priced brokers with excellent services.
With Tradestation you get a top notch trading platform - if you have a decently sized account ($100,000) or you trade frequently the platform is free - that on itself is an amazing deal. As a broker is good but not as good as IB.
With IB with a single account you can trade equities, futures and forex. With IB you can trade plethora of markets worldwide. Commission are better - it depends how much you trade, but it could make a big difference if you trade frequently enough. With IB you can put and take money in and out of the account very easily online - with TS, banking it is a bit of a pain. IB margins are also better.
But I find the IB trading platform horrible. You need an extra software to trade - that can be expensive if you want to match what you can do with TS. Plus you may need historical data - TS comes with plenty of it. So if you get something like eSignal or Metastock the money you spere in commissions may go in software and data. Ideally you should have both - and that is what I do. Use TS for longer term position a keep over $100,000 to have the platform free and use IB for short term trading. If you have to pick one I would say IB if you day trade TS if you swing trade or longer.