No matter what you pick, right now Triumph sports cars are probably one of the most under-valued british sports cars on the market. The cost to restore a triumph right now is far higher than the cost to buy one already restored. I personally dont see this trend lasting for too much longer, once the economy turns around, but I say buy the best you can afford, mo matter what car you decide to go with.
Myself, I went with a real driving restoration project TR6 which I enjoyed for one summer, and now I've got completely torn apart. I paid 4k for my TR6 and figure i'll have 20k total into it by the time i'm done restoring it. Would I have been better off spending that 20k upfront and buying a very nicely maintained / restored car? Probably.. but I'm enjoying tearing this car down and fixing stuff, and putting it back together with the changes / mods that I want to do to it. As said above, you need to decide up front do you want to work on the car yourself? or just drive it?