I recently purchased an 8-quart stainless steel Presto pressure cooker ( http://www.amazon.com/Presto-8-Quart-Stainless-Pressure-Cooker/dp/B0000Z6JIW/ref=sr_1_6?ie=UTF8&qid=1363975444&sr=8-6 ). I have loved its results on everything I've tried it with: stocks, bone broth (it gels like no other), risotto, beef shanks, pork shoulder... (pressure cooked carnitas are the bomb). I may try the caramelized carrot soup tonight. Haven't tried beans yet either, but I'm sure they'll be awesome.
The reason I went with the Presto was that I wasn't sure whether pressure cooking was going to be a technique I'd use on a regular basis. I wanted something inexpensive, but not cheap - something that was solidly built but not boutique. This Presto fit the bill. Solid construction, heats evenly... easy to use and clean. Nice! Now that I've put it to the test, I'm a believer in the power and convenience offered by pressure cookers. Part of me wishes that I'd splurged on a Kuhn Rikon, but those things are four or five times as expensive. I don't think I could justify that given how well my Presto has been performing. I may pick up a 4 or 6 quart KR one day to use for smaller dishes.
Concerning size, I don't think I'd get anything smaller than a 6-quart cooker, especially if you're going to make stocks or stews based on large cuts of beef.. Throw a chicken carcass or a pot roast in there and it can fill up quickly. If you're concerned that it might be a giant unitasker taking up valuable shelf space, you might try thinking of it as a nice pot that also has a strange lid which transforms it into a vunderkontraption. The LID is where the magic is. And lids store easily. If you have a nice pressure cooker, you won't need a stock pot; you've got it covered. Of course, having both doesn't hurt!