I sailed on the Ocean Adventurer a few years ago. A wonderful ship although I think that this is her last year w Quark.
I will add this to the people who have spoken of the value of the years of experience and size of the expedition staff that Quark has over almost every other Antarctica outfit in the business. You also have to consider the backgrounds of the staff. On my Quark cruise one of the staff members was a Russian who served for several years as chief of Russia's Vostok Station in Antarctica - look it up in Wikipedia. The stories he told and the experiences he had were great although he was not there and would not discuss the Vostok Station murder.
Of it were me I would go with Quark. There are some very good travel agencies out there that do just Antarctica cruises. I worked w one of them when I picked my South Georgia Island trip. However according to CC rules you will have to find the right one yourself as we are not allowed to recommend any travel agencies.
DON