No-flip mattresses are the latest gimmick manufacturers use to limit the life of a mattress and force you to buy a new one much sooner than you would have to with a high quality double-sided mattress. No-flip mattresses feature an innerspring unit with several layers of foam--usually cheap low density PU and sometimes visco of unknown density on one side of the springs only, all of which quicky compress and soften to the point where the mattress is no longer comfortable.
I would absolutely agree that 2-sided mattresses are preferable to the no-flip variety. What you should look for is a sturdy innerspring unit made with heavy gage coils and as little built-in padding as possible. It's better to add you own comfort layers which can be tweaked, changed or added to any time your needs change without replacing the mattress.