I cannot comment on the C70 but have a T5 and its well worth getting. Just make sure it has been well looked after as these cars need a bit more TLC. So a good service history, preferably with the dealers is a must.
Depending on the mileage you do, you might soon need a timing belt at that mileage some get them done between 80k - 110k, usually 80k if anything has been moded.
If you are only after the convertible then obviously the C70 but if its more the driving because they all have similar front end's I would look towards the C30 or the S40. S40 has a lot more space.
Good luck
