'Approval through rigorous testing' I think. Wheels that are advertised as 'style' are cheaper as they haven't been through the testing and approval procedure. Someone in the know may correct me.

You invariably get what you pay for. Me with mild OCD would go OEM to be sure.

***EDIT***

Not alloys but as an example from another thread........

otter-spotter wrote:

I bought a set of genuine Wolf rims from Craddocks (actually a set of BFG AT's on black Wolf rims). After doing some research they seemed like an OK deal.

However, be aware, they do sell "look a like" versions which are cheaper, so make sure you specify exactly what you want. Even though I specified and paid the premium for Wolf, what was delivered were the cheaper versions. It was pretty obvious, the cheaper ones don't have the right stamps on them (I was after ANR4583PM) and, when I ran my hand around the inner rim there was a noticiable indent (joining seam?) which is not on the Wolf rims.

Anyway, I phoned them up and, to their credit worked out from their end that I had been sent the wrong ones and corrected it pretty quickly. The replacements arrived on a pallet and were padded out with box's that I think the rims had come to them in - looked like Land Rover OEM, so black Wolf rims from the factory!

I guess you can get cheaper by looking at surplus places, but this was for a new Landy and I wanted to get something that looked smart and had a known history.