Re: The choice
Well I agree with your comment about quality, and I have personal experience with a 236 and 256 SSi. Great boats, but I think I favor the Cobalt. There is something about them that just sticks with me. Always has and I like the 232 a lot.
If you want to go 50+, go with the 8.1 no matter what they tell you

Second choice would be either the 320 5.7 Volvo or 320 6.2 Merc, but I'd want the 375. The 8.1 375 Volvo pushes the 256 SSi to 53 MPH with a light load and trimmed out. A 220 Cobalt will barely make 50 with a 320.
Edit: Also depends on dealer strength where you are. I don't like the local Chap dealer, lots of BS and cruddy service. The Cobalt dealer on the other hand has a very good reputation.