Need advise on which new truck - 1/2 ton or 3/4 ton?

I think your Tundra would probably be fine for that size trailer so I'd personally give it a shot and if you're happy with it, keep it. If you were buying a new truck regardless I'd tell you to go with an HD model because you get a lot more truck for about the same money and it gives you more flexibility with minimal downsides.