I walked the East (southerly) side about 10 years ago and found it tough going, not because of river crossings, but because the 'path' that followed the shore of Loch Etive was a sandy, pebbly 'beach'.
The late Nev, a Challenger of great renown, and Ann Tandy followed t'other shore that same year and they reported that it was easy-peasy.

Walked that path from Glen Noe through to Glen Coe in October. As John remarks it's tough(ish) going and was very boggy in parts. Sorry can't help you re. the Allts ,would imagine they would be tricky in spate.

I should have added that I walked from Taynuilt to the north end of Loch Etive, camping around Dalness in Glen Etive. Camping spots were hard to come by.

I walked along Loch Etive in 2012 https://blogpackinglight.wordpress.com/2012/05/20/tgo-challenge-2012-part1/

Many thanks for the helpful replies and your experiences - as a result, it will be West Side Story for me in May.

I did the west/north side in 2009, although i did have a route planned for the east side if I felt up to it. The west side was pretty straightforward, although obviously not as many hills to get a look from atop of.

I should note that after Glen Etive I went north through Altnafeadh instead of NW to Glencoe, but at that point East and West will have already merged, unless you would stray further west towards Achallader or Orchy.

PS Loch and Glen Etive are both beautiful!