Sorry - can't second Carradale - I think there are much better beaches and the 20 mile drive down the single track road is a pain. Campbeltown is the pits too. And once you're on the Mull of Kintyre your stuck there - too many old people for us!!
I think if you want the "real" west coast (hills, white sandy beaches) you need to head north of Glasgow.
Mull though gets my vote - lots to do, Tobermory is a very cultural town, great beach at Calgary. Iona day trip... what is not to like! Only 45 mins on the ferry from Oban. Personally, I don't think there is that much to do around the Oban area.
Arisaig is also fab - the best beaches on the mainland I think. Beautiful white sands. Close enough to Fort William not to be too isolated. you can also get to Skye easily via the Mallaig ferry.
Other islands you could consider, if your into that kind, of thing are Coll (no town though great beaches) and the Western Isles. Barra is the most beautiful I think though getting there is a big commitment - 6hr + ferry from Oban.
Do not underestimate the Scottish Midge - they really are bad. We went camping in Ardnamurchan in last August and they were in full effect...