In my experience, there really is no rule of thumb. In some cases, the aftermarket equipment is BETTER than the OEM, and in others the OEM is better. Regarding Land Rover, I do not have enough experience with Rover parts to give you good advice. In your case, I would recommend either finding an answer on Land Rover web boards, or just going with the OEM products as an insurance model. But it appears you can save some money by purchasing from BP or BA as dealers tend to mark-up OEM equipment higher than those independents.
Let me relay a recent story.... I am a big Porsche nut, and have a track garage and a track-only 911 GT3. One big expense is front brake rotors. They cost $300 each from Porsche Motorsports, and last 6-8 track days. Stopping power is definitely a 10/10. That is an expensive consumable! Anyway, some of the Porsche track guys on the web boards learned that the Cayenne SUV runs the same size front rotors as the GT3 Cups (race cars) and GT3 street cars. Through a non-OEM manufacturer, they cost $100 each. Turns out they last 12-14 track days and are rock solid (8/10 in performance). This past summer, the hobbyists bough out the entire supply on Amazon and eBay!
Every car brand seems to have their little quirks...
-B
