Easiest way to check would be to look at one of the well known online merchants - Barlows or Messicks - and see if there's a material difference in price.
As with anything, OEM is a known quantity. Some aftermarket parts are excellent, some are not. Some types of part are more important than others - I'm not sure I'd go out of my way to have an OEM hydraulic hose, I might for engine parts.
If you don't have the experience or confidence to know which parts you care about, or to work out which after market providers look reliable, I'd say you're safer to stick with OEM parts. That even extends to things like fluids - they're not well standardised, and in the end I decided easier to run SUDT in my BX.