Project Vote-Smart is a non-partisan website that tracks voting records and interest group ratings. Interest group ratings are your best way to gauge a politician's positions because the organizations you know about, who care about the same things you do (or who you despise because they pursue the opposite policies) do the work for you.
So, go to project vote-smart and look up interest group ratings on both Sestak and Specter, and make an informed choice of which best represents YOUR interests. (and that's not you as in uberblonde but the generalized you, of course)
http://www.votesmart.orgOne specific item I'll point out: in 2004, Specter advocated for a flat tax and elimination of the estate tax.
http://www.votesmart.org/npat.php?can_id=53340#838 Since when are THOSE progressive positions? Specter also voted for DOMA.
If you care about the environment, also look up the detailed scorecard at lcv.org (League of Conservation Voters). Check Specter's lifetime score, not just the last year since Sestak challenged him in this primary.
edited to change confusing subject line.