orbi or extender which one is best

The R8000 was released in 2014. I'm not saying its bad or has issues, but its getting a little older and I'd be curious about adding to it value wise. While I think an extender might work perfectly fine for it, I'm not sure how much extra I'd invest into keeping it going with adding an extender to it.

If you were going to add an extender, I'd look for either an EX7500, EX7700, or EX8000 to add to it. You can find them used/renewed for pretty reasonable prices, and they're triband extenders. So, they preserve the throughput (speed) of the system versus being a dual-band system. The single/dual band systems take a significant speed hit by the very nature of how they have to work (around 50%). 

If it was me, you can pick up the RBK763 renewed for around $250 on Amazon. It's a 3-pack of 2 satellites and a router, and it is a triband system. It'll have much better coverage (maybe to much based on home size) and will get you to the next generation (AX) of wireless versus sticking with the AC speeds. Plus it'll have wpa3/wpa2 mixed mode for increased wifi security (not that much better). Plus the orbi mesh systems are rock solid in performance and stability. While its a renewed system, amazon sells 3rd party warranties if you're concerned about it that still put it significantly cheaper than new.

I compare it to a car a little bit. After a certain amount of time, the parts start nickel/diming you. Say you do buy the extender, and then the router craps out. Do you invest more in the same generation router or invest in the next generation? But then your extender might not match. So with a car, at a certain point, you get either a new car or a decent used one with lower miles. This would be similar.