If there are preservatives in the tinned ones it should say so on the tin. As for the syrup... I have no idea what that'd do.
Canned stuff generally does without preservatives as far as I know, the way they preserve it is seal it in the tin, then heat the whole thing to pasteurise it. The possible problems I can forsee would be:
- syrup creating strange tastes
- the pasteurisation changing the flavours somehow
Really though, seeing as you're using sucrose as the fermentable anyway (well, that's what I did) the syrup shouldn't be too much of a problem. I suppose the other side of the argument is that this is a pretty expensive brew as it is, so a few extra dollars for the good fruit might be worth it. Hope that helps!