As someone who uses Mental Ray, Vray and Maxwell on a regular basis I thought it worth giving my 2 cents. First here is my order of preference depending on need/feature.
IBL (image based lighting with HDRIs):
- Mental Ray
Producing good, fast diffuse metals:
- Maxwell (resolves beautiful glossy reflections, depending on the scene fastest of the bunch)
- Vray & Mental Ray tied (neither have a real edge here, both can get the job done, so depends on the scene)
Animation (moving objects with GI):
- Maxwell (no light caching and no chance of flicker of any kind)
- Vray (gotta cache that light, refining those fine details in a lighting solution without flicker is still an issue)
- Mental Ray (same but GI+FG workflow is a bit more cumbersome than Vray’s and honestly I find vray’s GI modes are just easier to deal with)
Animation (static scene with GI):
- Vray (unless IBL only then I’d say Maxwell)
- Mental Ray
- Maxwell (unless exterior, then I’d put it at 1 or 2 depending on the scene. Since flicker isn’t an issue with static scenes we loose Maxwell’s strength in this case)
In camera Motion Blur & DOF:
1.Maxwell (by far. You get both practically for free in Maxwell)
2.Vray (Does well at both at the cost of render time. Doesn’t cripple the render or anything but it ain’t for free)
3.Mental Ray (Both MB & DOF still in need of major rework unless you’re using iRay)
Direct Light only stills & Animation:
- Mental Ray (I still find look dev with direct light a pleasure with Mental Ray)
- Vray (Could be 1 depends onthe day, unless in camera MB & DOF are desired, then 1)
- Maxwell (Not maxwell’s strong suit)
“Maxwell is Slow” - Not really. In fact it’s faster than Mental ray & Vray depending on the scene. Exterior daylight & IBL cook along in Maxwell. Studio lighting setups can also be quick. Though if you’re talking an architectural interior with 120 IES lights then maybe not so fast. It depends. And to the comment comparing Vray’s brute force to Maxwell, I don’t agree. There is no comparison there. Maxwell is built around an unbiased core and optimized for such. Vray brute force is just forcing a biased engine to behave like an inefficient unbiased render engine.
“Mental Ray Sucks” get Vray - I love Vray but as a competent Mental Ray user of many years I was not completely blown away when I started using it. Vray’s light engine is what sold me on using it, but honestly I was disappointed that glossy reflections were not really any faster. Also soft shadows were much slower than I had hoped etc.
"vray is a Magic Bullet, everybody’s using it! " - both Vray and Mental Ray take a lot of optimization to squeeze out great performance. Simply using it doesn’t make for fast or clean renders. I would say for a novice Vray should be easier to pick up. But that’s assuming that novice goes to make a cool IBL image for their first render. Piece of cake there! That said there are some sweet spots Vray hits just right. The physical camera is awesome! Vray lights are clear and easy to use. Passes in the frame buffer is pretty great!
Conclusion: I currently use Vray for the majority of my work due to it’s great flexibility, but I still use Mental Ray on occasion. Maxwell has been in my tool box since Beta and I use it occasionally as well, though I plan to use it more in the future as it has greatly improved over the past few years.
P.S. Just realize this isn’t as relevant to the OP as I had thought it would be. Oh well. If you want to do product viz with minimal pain. I would go Maxwell coupled with http://www.hdrlightstudio.com/