I agree that the San Juans plus Vancouver Island are likely to be a nicer trip (I live in Victoria and never tire of promoting the area) but the Okanagan is not a bad trip either.
Osoyoos to Penticton can be done on bike paths about 2/3 of the way. The middle part is on the highway but traffic isn't too bad. The trail beside the lake the last third of the way into Penticton is wonderful.
From Penticton you can follow the Kettle Valley Railway trail (part of the TransCanada Trail) as far as you want, east or west. I understand that the bridges through the Myra Canyon section of the trail near Kelowna have been restored since the big fire a couple of years ago. If so, don't miss it, it's a wonderful ride.
I'd be less sure of riding the road between Penticton and Kelowna. The shoulders aren't too bad but there is a lot of traffic and it's much hillier than the trail. I haven't actually ridden the road, only driven, so it might not be too bad.
All that said, if I had to pick one I'd go with the west coast.