I'd pick 8MP at 7 FPS. 7 FPS still captures pretty much all that is demanded from a security cam in most situations.
That said, higher frame rates are a lot more pleasing to watch, but things don't really start resembling smooth until you hit 15 FPS. If it could do 5 MP at 15 FPS, there would be more of an argument in favor of choosing 5MP. But 12 FPS would still look more choppy, giving it less of a perceived value.
I don't really care if a higher frame rate is more pleasing to watch.
Though it's obvious that lower frame rates have problems capturing fast moving objects/people. And that's why you said "... in most situations".
Yes. I even ran some of my cameras at 3 or 5 FPS years ago when my system was more resource-constrained, as I figured that was better than lowering the resolution.
The main argument in favor of higher frame rates is being able to capture quick motions better, like a guy in a hoodie that might only glance at a camera for a split second before looking away. At 7 FPS you'd probably capture his face once or twice, but at 30 FPS you would have 4 times as many pictures of him at slightly different angles. The thing is, appropriate lenses (zoom) and camera positions are going to improve your chances of a good capture a lot more than just making more captures.