Spezza for sure. You have to remember that this is basically Jason's rookie season. He has a very long way to go and he will improve so much over time. Havlat will improve too, but Spezza's potential is just so much greater.
I think if we were talking points, I'd give the potential edge to Spezza. I'm not particularly convinced, however, that Spezza has more than maybe 30G in him. I'm somewhat more convinced that Havlat has 35G+ potential.
It's far from a clear-cut situation, though, and I think it will laregly depend on who those two eventually play with as regulars.
We're talking about a guy who has already scored 15 goals in what is his de facto rookie season. He is on pace for 25. More importantly, 11 of his 15 goals are even strength. That's more than any Sen other than Hossa & Alfredsson. Wait until he gets on the first power play unit. His accuracy is incredible.