When you say Universe Starscream is a different mould, you mean the toy that is usually called Masterpiece Starscream, which was released in Universe packaging in the West. It's a completely different scale to Generations Thundercracker, and is only useful scale-wise if you want to reenact this:
If you do, and can spare the money, then get it over any other modern Seeker mould - it's a good toy. If not, then just get the cheapest Classics/Universe/Generations seeker you can find that's not a knockoff or Ramjet - they're all basically the same mould so if you don't care which one you have, go by price. It doesn't matter - just get the cheapest.
It's "capisce".