Pretty hard to take Seguin here, unless youre in a pool that factors in salary, in which case I'd take Seguin. I think Seguin will be around 70-75ish point pace and Spezza PPG pace (82 GP pace, for both).
Future Seguin, this year and maybe next year Spezza.
For Fantasy League purposes I think Seguin is the better pick depending on what stats are included. I think Seguin will score more goals while being close in points so if your pool values goals more then I'd lean to Seguin. My Yahoo pool includes GWG and between thinking that Seguin will score more and that Boston will win more games and that Seguin is listed as C/RW.
If it's straight points I'd take Spezza. Seguin may outscore him this year but it wouldn't be by enough to take the risk as Spezza is the more sure thing.