BMW Warranty Extension Not an Option
"Sticking with BMW" is not an option for extending your Warranty. [unsure] BMW Warranty Extensions are limited to original owners only! To do otherwise, BMW would shoot their CPO Program in the foot.....and they're not about to do that.
You can extend the BMW Maintenance Plan while you're still under the OEM Plan regardless of whether you're the original, or the nth owner. At a minimum, it'll cover your 60K-Inspection 2, 75K-Oil Service, 90K-Inspection 1, and a couple of sets of brakes, brake flush and coolant flush, probably. If you do all or most of your own maintenance, it may probably not be worth it. Since you're even considering it, I take it you're not in that category. I added the Maintenance Plan Extension to my CPO. Just turned 54K and had Inspection 2 and brakes done for nada. So I've basically broke even with 2 1/2 years and 46K to go. But then again, these numbers are based on dealer rates which are usurous, at best.
My opinion of the non-OEM Warranties is that a lot of them seem like glorified Ponzi schemes....a lot of sales then bancruptcy. The health club syndrome. I've never had one though and many have been satisfied with their policies.