The Orkney's were lovely but I have to say I enjoyed Shetlands better. We travelled via air from Edinburgh to get there. Alternatively, you can travel by ferry from Aberdeen. It is an overnight ferry run by NorthLink. Because the Shetland Islands are very long, I broke the week trip into 3 different hotel stays. We focused on the southern section, the northern section and then middle (Lerwick). We travelled by ferry from Shetland to Orkney to spend a second week on the Orkney Islands. Since the ferry left from Lerwick, we did the middle section last. The visit Shetland webpage shows a good idea of the different sites to see. The only thing we didnt have time to see was the Fair Isle. I would recommend August. We travelled in September and missed the puffin migration by about 2 weeks.
Since Orkeys's are closer to mainland, they get more travellers. That is part of the charm of Shetland. People were just amazed that we went all the way there and spent a week on the island. But the natural landscapes, historical sites just couldnt be beaten.
I stayed at the Sumburgh Hotel, Baltasound Hotel, and Lerwick Hotel. All lovely and different.
I hope to go back in either 2018 or 2019 but to also do the Hebrides. Hope that helps