I'd buy the TT bike then make the decision. If you don't use it then at least you can blame not using it if you have a worse than expected bike, and if you do use it and it goes well, you can congratulate yourself and so on. Buying it is really a no lose situation. More kit always trumps training in my house.
PS. There's a bargain TT bike in my LBS. My wife uses arguments like I still owe her for the last bike I bought as a reason not to buy it. I'm going to have another look at lunch.