well..its always hard for me to decide or talk about render engines because i have my real personal opinion and because there isnt the best tool..
That said, please take everything i say with a grain of salt
Mental ray, i dont even wanna talk about this disgrace, its dead, developers fucked up on a 360° in a colossal manner and it deserves to be dead.
Maxwell Render: i could probably talk for hours about this
For me it is the state of the art render engine on the market. There isnt anything that goes close to its quality, but it has a very steep price which is the speed and the fact that being a freelance you wont be able to pull out a single work without a renderfarm. Materials/lights/realism is unsurpassed on maxwell, also the ease of use, the amount of materials present on libraries and the material editor web online resources is outstanding, it gives you almost perfect materials on the go, you have just to swap textures, make some adjustment and hit render.
Unfortunately the render times sometimes goes too high, i have no clue what Next Limit is going to do in the future but at the moment is extremely hard to use for normal project unless you use render farms.
Redshift: eh..impossible to resist, speed is insane, fully integrated, i had some quality/shading issue with the first RS architectural material because it resembled too much Mental Ray shading, and i dont particulary like that, fortunately they completely rewrote the shader and it will come on 2.0. The new shader gives you way more realism and you can use different brdf shading models, its an extremely nice monolitic material.
On top of that GPU for freelance and small studio is hands down the best investment, and its gonna be the future. Redshift is at the top now because developers are extremely talented and they deserve to be there, they listened and they worked hard. I'd recommend this engine over anything else at the moment, infact i've bought it right away.
Still i'd love to have Maxwell quality with Redshift speed, but hey, daydreaming isnt forbidden
One thing to let people understand (consider also that it highly depends on artist skill and sometimes i suck!), just a stupid comparison because that is what it is, but if you take this image i've made with Maxwell:
you have no idea how easy was for me to do that, i basically setup a glass material from the material wizard, with a green tint absorption, made couple of metals material, slapped a hdri, close up with camera > enabled multilight > hit render. Now if i were to try and replicate that image with Redshift, i suppose (i tried briefly but then i had to do other stuff so i'll let you know better when i'll try seriously) it would take me a lot of time to achieve that look, especially on the glass/light/scatter/caustic/light dispersion and realism, which that image is all about.
And this is an example why im so torn between Maxwell and any other render engine
Bare with me Of course another artist might do the same image in few minutes with Redshift or Mental ray, but i doubt i can with the same simplicity i have done it with maxwell. I hope i was clear and honest enough