I use tinned in my base sauce then fresh in a finished curry if i'm making a bhuna or something, the tinned ones can be a bit acidic but when they're cooked through in a base sauce it takes the edge off them and they become sweeter.
If you use fresh try plum toms, or toms on the vine they are bit more expensive but taste a lot better than the watery salad tomatoes the supermarkets sell loose.