As it happens, I've got both of those. They're both good cranksets. You're not going to install it before replacing the chain though, right?

The Gossamer is better looking and probably a bit lighter (if either of those things matter to you). The Tiagra shifts better. The bearings on the FSA bottom bracket wear out quicker than I'd like, but you can replace them with a 7900 Dura-Ace bottom bracket for around $25. I believe with the Tiagra crankset you need to purchase the bottom bracket separately.

If you aren't hung up on buying from a US vendor, you can get the Tiagra crankset dirt cheap here:

http://www.ribblecycles.co.uk/sp/roa...et/shimchar765