The 28s a decent coastal boat, but I've always considered the galleys very small for an otherwise fairly roomy boat.. She has the sweet C&C sheer and the classic good looks of that line.
The 30 is quite a bit more boat.. similar to a Ranger 29 (another Mull design) that's a good coastal cruiser too.
I think the Ericson line, (there's an older 29 from the 70s too, and a couple of different 30 footers) would in general be a cut above the other two assuming you found a good one. As with all boats of that age look for problems in the deck coring, rainwater leaks at various deck fittings and portlights, if nothing serious pops up at first blush then get a survey and go from there.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)