Buck,
I've asked that question to myself so many times it's unreal. So, now I just take along both hippers and knee boots on moose hunts. If you want a recomendation for the knee boots, - I have the LaCrosse Alpha Burleys w/1000 gr thinsulate. I could not be happier with these boots. I have worn them in Alaska glacial streams, they banks of the Yukon river, and countless days hunting here in Wisconsin for deer, turkey and hogs. They keep my feet warm too. They are more comfortable than street shoes to me. In the last two years I would estimate I have worn these boot at least 75 days, probaby more, and they still fit like a glove. If I were to loose them somehow, I'd go right out and buy another pair.
As for the hippers, I bought the Cabelas Ultimate hip boots w/armor flex 1,000-gram Thinsulate. I have only a limited amount of time in them, but so far I like them. They are going to Alaska with me in 4 months.
If I were forced to take only one pair, I would opt for the hippers, as I feel they are equal the the Alpha Burleys, but obviouslt they will allow ya to get through deeper water.