They usually claim that a stamped deck has a better air flow due to the forming of the deck by smooth rounded edges, curves, etc.. Keith is right about welding on the stamped decks. Usually stamped decks are a lighter gauge steel. Toro, several years ago, added a welded bull nose to the front of the deck for added strength, durability, and reduction of front deck blow out.
Welded decks usually add chambers, baffles, etc... to achieve a good air flow. This is why welded units usually weight more than a stamped deck unit.
My Toro 44 has a stamped deck that has worn down on one end from rubbing concrete curbs and walls. This would not happen as quickly on a thicker gauge steel welded deck.