Well. The general rule of thumb is aperture rules. You'll be trading off light gathering ability and resolution.
I've never looked through a 5" mak-cass but the extra aperture of an 8" would definitely give you something extra on DSOs and resolve greater detail on planets when the seeing allows.
Otherwise we'd all get rid of our 8s... 9.25s... 10s and 12s etc and race out and get a 5"
Although the optics of the smaller mak-cass are less affected by factors such as seeing etc and most probably offer more contrasty views?
I'd recommend getting along to a viewing night of a nearby astro society to get a look through a variety of scopes for a comparison