I've owned both omega's and can recommend both watches.. If it's going to be your only watch though, I think the Seamaster is the better choice - simply because it has a date. If on the other hand you have any use for a chronograph function the speedmaster is a great introduction to fine watches.

Never had the Tag on my wrist, but looking at it in the shop it just dosn't look as well finished as the seamaster.

Let us know what you decide.