You will have to sight in the cantilever mount after you've had that barrel off and back on the receiver. You'll also have to sight in the receiver mount if you take the barrel off and put it back on again.

Re: the saddle mounts, they're a great option, but they can leave rub marks on the sides of the receiver. I can get you a brand new, machined aluminum Aimtech saddle mount for that 870, with integral scope base, for $38, delivered.

Re: 500 v 870 in general, when you take apart a 500 for thorough cleaning, you have a whole heap of parts. When you take apart an 870, other than the forearm/slide, you have only two parts: the bolt assembly and bolt plate. More parts = more things to go wrong. I'd stick with that 870. I've had one for years and never had a problem with it.