Economically if you've never played Sins before Rebellion is the best choice. You get everything in Trinity in Rebellion. The only thing is of course Rebellion is more expensive in absolute terms, but you also get the most content.

Perhaps the only possible issue is that Rebellion requires the Steam program to play while trinity doesn't, so if for some reason Steam doesn't work well for that might be an issue. Also Rebellion will be a digital download only so it could take a while to get on a slow internet connection.