We were in both Shetland and Orkney in 2022 in the motorhome, with my bike on the back, and loved both of them, although after Shetland, Orkney can feel tame and very small.
I had some some great rides, though there are downsides - lots of weather (constant wind, frequent rain) and the fact that they're islands means a lot of dead-end roads and loops to do any distance, as well as some steep hills on the most scenic little roads.
Shetland is bigger and has many corners that feel remote, although some of the main roads radiating from Lerwick can be surprisingly busy. Do try and get to the northern islands of Yell and Unst, which is easily done on frequent ferries. The views of bare moorland and coasts throughout the archipelago can be magnificent.
On Orkney, the main road from Stromness to Kirkwall is busy but few others are. The ride to South Ronaldsay over the Churchill Barriers is a must, as is the north west to Birsay and Skara Brae (and Ring of Brodgar in the centre, though you'll be dodging tour coaches). A day on Hoy is good too. I didn't visit the other islands - some sounded appealing but they are very small and wouldn't offer much cycling.
This is as far as you can go