i just showed up in ushuaia and bought tickets on the spot in Ushuaia for a ship on Quark Expeditions from Turismo Ushuaia for the same price as they normally cost from Quark (i believe it was around $3,000-4,000 USD). Quark doesn't offer discounts, but Turismo Ushuaia offers free winter jacket/pants/boots rental with the cost of a ticket from them (also note info on them at http://www.virtualtourist.com/travel/Antarctica/Packing_Lists-Antarctica-BR-1.html). note you need to have travel insurance in order to get the ticket -- i recommend buying a policy from Seven Corners before you leave. they are cheap and have great coverage, i used them for my whole trip. but you cant get it after you leave, so plan ahead.
if you speak fluent spanish, some of the other ships might be more young/party oriented, since Quark's crowd tends to be older (read: the average age is close to senior citizen age), but they are top-notch with regards to ship quality, number of landings/day, briefings and lectures. and theres usually enough younger folks as well along to keep it interesting. and they have cool hikes etc. I strongly recommend Quark.
