Are you going to be touring or doing day rides? How many days do you have for cycling? What are your normal daily distances? Is your start point determined by train stations or are you leaving a car somewhere?
Arran is good but you can get round it in a day, or two at most, so you might want to go on from there to Kintyre etc. or even Islay depending on ferry schedules.
Mull is excellent and would probably be my choice, bit further to travel to Oban than Glasgow though.
On the mainland starting just out of Glasgow a good tour would be up to Aberfoyle, round Loch Katrine then on to Callander, next day take the cycle track to Killin then minor roads up Glen Lochay and down Glen Lyon to Kenmore, day three over Glen Quaich and Sma' Glen to Crieff, Comrie and Loch Earn, day four back to Callander then Thornhill, Kippen, Fintry, Lennoxtown and back to Glasgow. Total distance about 220 miles.