Where I work it usually pretty simple, If the machine is one of the brands that we sell, it will almost always be an OEM part. When it comes to other brands of equipment than we usually go after market, which usually means Total Source, since this is a very big company with a huge volume of parts sales T/S gives us a preferred customer discount (or so I am told, I'm a roadie, not a parts guy).
As a road tech I've learned that there are some parts, and some machinery brands that simply must be OEM no matter what the cost, and when I speak to our parts dept. people I make it very clear that I want OEM only.
We roadie's used to get a lot of resistance about the demand for OEM parts since it was cutting down our purchases from T/S, but after a few catastrophic failures related to poor quality after market parts, our parts dept manager finally realized that the roadie's do, in-fact know what's best.
There are times when after market is better too. For example, the radiators that we have been getting from our OEM sources over the past few years have been complete junk, and don't have a very long warranty. So for a few more bucks and twice the warranty length a radiator from T/S is a better deal for the customer despite the fact that it probably came from the same factory as the OEM source version.
I usually don't discuss the source of the parts with my clients unless there is going to be an extended delay in shipping or an outrageous difference in the price. I explain that the after market parts for this particular job are known to be of inferior quality, and that it will be penny wise but pound foolish to go the cheap route this time. That and the fact that the part may have a warranty, but the labor doesn't will usually make a hesitant client see things my way.