Safety should be your first choice. If they are equally safe, then I'd choose the roof that fit your budget accordingly. Most people would just choose the cheaper roof. It's up to the buyer to decide if 8 years is good enough for them. If the roof is new and safe, most buyers would be happy with that. I'm sure their inspector would point out what kind of roof it is, its condition, and its estimated lifespan.
I'm a big fan of EPDM rubber roofing, and that is mainly what my company installs on flat roofs. It should be a decent amount cheaper than TPO and have the same lifespan. If someone ever tries to sell you on TPO over EPDM because TPO is white and reflects sunlight better... They do make EPDM in white. Everyone always thinks the difference is TPO=white and EPDM=black. This is not the case.