You're thinking about this as black and white, right way vs wrong way. This, as with buying players on the TL, setting arena prices, etc. These are gray areas, there's not right or wrong way, its the way you think is best for your team.
I don't doubt that L6 trains faster then L4, and I admit I don't have a ton of data on this, just my own experience and seeing other peoples similar situations. I would gladly buy a L6 trainer, but what I would pay is a lot less then what others pay. I can use that money for other ways I'd prefer to benefit my team.
"Well, no ones gonna top that." - http://tinyurl.com/noigttt