I'm carrying on from Paisley so would expect it to be late in the evening. Is the A82 likely to be faster than the A814?
Talking from experience of having ridden both roads many times the A82 will be marginally faster, as there is less climbing and it's shorter, but as far as pleasantness goes the A814 is better (IMO).
Aye, A814 definitely more scenic, Loch Lomond is just a big loch, the Gareloch at least has hippies, Submarines and ship fueling bunkers
Pleasantness it's dependent on your measure, if it's traffic, it really is a case of Day or Night, and most touring riders will be riding at Daytime where if on the A82 route I'd definitely tolerate the RCN40 over the road if insisting on going up Loch Lomond.
The camping ban has moved the Neds up to Loch Etive, so there's less chance of a pissed up weegie battering down the road trying to make Tesco before 10pm to get a cargo. Consequently you may get this on Glen Coe as they try to make Ballaculish Co-Op before Off-Sales end.
I think I've mentioned my journey across the A85/A82 at night before.