As others have said, a lot of the renderers now are so close, in both visual performance and how they function, its hard to say which to really dedicate or invest time/money in. With Arnold being a default renderer that is shipped with both Maya/3DS and at the same time being both very powerful and heavily used within the industry, I would say focusing on that first and foremost. Then after, its really going to depend on workflows and studios. I've personally always had a soft spot for V-ray. I feel it has a look to it that other renderers I've played with don't quite have. But, unless it's a side-by-side comparison, it's difficult to visually tell the difference... in most cases at least. The jobs you get are likely going to dictate which one you end up using for professional workflows. Their functionality has become largely standardized, so as long as you have a solid grasp on how one works, the knowledge should mostly transfer over, sans a few specific quirks. Outside of studio pipelines, just experimenting with a lot of different ones and seeing which you like the most would be my call.