In order for filters to work, they have to restrict flow be it transmission filter for automatics, oil filters, air filters, even fuel filters and cabin filters. Some cars also have power steering filters as well as primary and secondary fuel filters (diesels come to mind). It is essential that they be able to do their job so choose what the manufacturers have installed as OE parts. There's a difference between OE and OEM parts though.

OE parts come from the casa, branded with the car make and usually are more expensive. OEM parts are usually designed to meet the factory specifications and come from the same part manufacturer, however they're not repacked or rebranded as "genuine (insert car make) filter." For example, there are tons of filters by Bosch but some car manufacturers rebrand them as their own. Also note that not all filters, just because they're a certain brand, are always manufactured where they would come from. For example, not all Bosch filters are manufactured in Germany. Some are done in Austria, France, even India. In my experience I've learned to be picky as to where parts are manufactured from and that they're OEM parts at least, but OE would be much better too.

Most filters have a standard to adhere to and can vary slightly. Statistically, this is known as having error bars in graphs and such, where the mean represents the standard. It's generally okay to be within that range but if a test has been done and the filters do not meet these standards then by all means, run away from them.

The only situation I'd use cheap aftermarket filters is when I'm going to be using it temporarily and replace it sooner than it should be. For example, if I went to some place that was dusty, the air filter would clog up. Rotating it would not help, so any cheap filter that can perform better than a clogged one helps a lot, but replace it soon with the correct OE or OEM part.

K&N air filters have had an issue whereby the oil used to coat them prevents the mass air flow (MAF) sensors from working properly, or even damaging them. So keep in mind these things when modifying your filters.

In order of preference, I'd choose:

OE > OEM > aftermarket

But due to budget constraints have almost always used OEM parts for my w123. They've been wonderful, no doubt, and in restoring a car that's 25 years old and used daily I'm sure the OEM items work perfectly. There is a catch though, some manufacturers that produce OEM parts claim to meet manufacture standards. However my experience with *some* OEM brands sadly are not as great. When I had some bushings installed on my car, for example, the cheap ones were Febi-Bilstein, Moog, Meyle, and such. True enough, after about one and a half years or more of use, some of the newly replaced parts have already deteriorated. So even if OEM parts claim to be spec'd accordingly, ask around first.

In terms of cost, I'd choose:

OEM > OE > aftermarket

This is because even if aftermarket parts cost much less, they're not as reliable.

But in terms of reliability, I'd choose:

OE > OEM > aftermarket

Hope that helps