To be honest, as you are an islands and slightly off the beaten track person, I am more than a bit surprised that you are just trying to stay on the safe Rick Steve's forum beaten path.
Yes I am a big flag waver for the Isle of Gigha, and for very good reason, at least part of it is that it is a community owned island (like Eigg). To me it is a little gem of an Island which benefits from not being on the mainstream tourist trail.
And Campbeltown really is just a lovely town especially if you get somewhere lochside to stay. At least for me it also benefits from having an old time movie theatre.
In some ways it's a shame you can't connect straight there by air, but as you will be driving you can't start a road trip there, as only local rental companies. I could do that as I would be on the bus forward to Gigha, Oban etc.
I also personally actually prefer Bute over Arran.
There is a reason why Bute and Cumbrae were seaside resorts to Glasgow, if genteel ones. Both, especially Bute, also had (and Bute still has) an Italian influence, from the Italian diaspora who settled there.
The beauty of Bute is that there are three ways to get there, so the chance of weather stopping you is very small.
They are-
Wemyss Bay
Gourock (a standard diversion when Wemyss Bay is untenable)
Western Ferries from McInroys point then Colintraive
I would say Bute x2, Campbeltown x1 and Gigha x2. With an extra night you could add it to Campbeltown to sweep up the bottom end.
My personal island choice down there would always be, in order, Cumbrae, Bute, Gigha, Islay, Jura, Arran. The only reason Gigha isn't top is ease of access. Both Cumbrae and Bute (again in order) win out on that. Cumbrae really is a one nighter, unless you want to use it as a bolthole.