Either one will offer scenic.
I like the Stein Valley #12 as the road is mid way up the valley mountain side and not on the valley bottom so you get good vistas to Lytton; Spence's Bridge to Merritt is rangeland (arid climate) so sage brush desert scenery.
Merritt to Kelowna is 4 lane high speed highway.
This is my choice:
I would definitely go north from Whistler (I assume you started in Vancouver?) through to Lillooet. Then you need to choose between 97C through Ashcroft and Logan Lake or Highway 8 from Spences Bridge to Merritt. I've driven both of these, and they are both nice, but I really like the climb up out of Ashcroft and, as much as its certainly not natural, the drive through the Highland Valley Copper Mine is something else.
I agree with V67 re her suggestions...the Pavilion Lake road is spectacular as you climb up out of the Fraser Canyon and across the plateau. You then see some of the old Cariboo as you come down from Clinton towards Cache Creek. Having spent my teenage years in Kamloops, I have a certain fondness for the old highway 1 and find the sagebrush kind of interesting.
Realistically, you cannot go wrong on any of the suggested routes! Last time I drove the 97 Connector from Merritt to Kelowna, I saw a herd of deer and a black bear (this was very early in the morning mind you).
is this the road you are referring to? https://goo.gl/maps/2mSQgAwcw732 is that road paved? as google streetview does not have that seciton. The one usually mentioned that google maps defaults to is Highway 99 Duffey Lake Road.
FYI if it is a gravel road the rental car agreements would not allow travel on that road.
Fair question cbaarch...I am trying to remember from a few years ago. If it is, then the other route via 99 is still an option.
Gave it a second look using google streetview. It goes on the road from Pavilion to a cattle guard https://goo.gl/maps/Jv5X49SguC72 which shows a gravel road and some sort of sign about Private Road which is often what I have found on First Nation Reserve land.
When I first looked it had no streetview at the other end near Clinton.
It's a logging road, gravel, from Kelly Lake to Pavilion. We did that route two summers ago. The view coming down into Pavilion was worth it though, otherwise a little dull.
+1 though for the Stein Valley suggestion.
Hello Kristine F, Sierra from Tourism Vernon here!
We have many scenic sights in our city and it would definitely be worth stopping by! Kalamalka Lake Provincial Park (…gov.bc.ca/bcparks/…), Allan Brooks Nature Centre (https://abnc.ca/), and the Grey Canal Trail are all fantastic locations that we think you'd enjoy! Please feel free to contact us should you have any further questions!