Don't go for anything round, the performance will not be good enough to get any real air time. A very large rectangular tramp with 10.5 inch springs will give you a good bounce and plenty of air time and space to move about. An all weather two string bed would be much better than a black, solid polymesh bed ( polymesh is the type you get on round trampolines)
It might be worth investing in a bounce board, as the real snowboards can easily damage the trampoline bed ( the bit you bounce on)which is super expensive to replace.
What type of skills would you be practising or learning from scratch?
Do you do superpipe, freestyle?
Please Santa, make it snow in the UK!