I'd be surprised if the engines themselves were really different. I'd suspect that at least most of the time the quality difference is part user and part modeling program. Sketchup has a lot more novice users than 3ds max. Max is a beast to learn and is crazy expensive so is scares off most of the hobbyists. Also, you can add a level of detail in Max that is rarly seen in SU, and often those details make all the difference in renderings.
-brodie