where to spend more time at the end of our rick steves turkey trip

That's completely up to you. Of course in Istanbul is much more to see. Or you can go by ferry to Greece. Samos island is very close to Kusadasi. You can even see it from there. Then from Samos to Mykonos, Santorini, Athens. That's how we did it. You can combine it and add islands as you like it.

As already mentioned, it depends, of course, on what you are interested in. We left Kusadasi directly for Samos and then to Patmos, Kos, Santorini, and Athens. Once you get to Samos, there are lots of island options by ferry. Or you could head back to Istanbul ....

Unless you plan to spend time in Istanbul before your tour begins, I'd suggest spending a couple of days there after your tour. There are more interesting sights in Istanbul than the tour covers. I spent 2 extra days in Istanbul before my Turkey tour and was glad I did.

I'd opt for more time in Istanbul too. I love that city. I spent 8 days there on the RS Istanbul tour, and then 2 more before another tour (the Villages tour) and I want to go back again. There's just so much there.

We took the RS Turkey Tour a couple of years ago and we certainly enjoyed it and are currently advertising it among our friends. We went to Istanbul 2 days before the tour started, spent two days there on the tour, and when the tour was over we returned to Istanbul for another 5 days = 9 days total.
We both agree that was a good move and we also agree that we will return there in the future.

We don't do tours but we did spend 5 nights in Istanbul last spring when the tulips were in bloom. There is SO much to see and do in Istanbul I'd suggest at least 5 nights either before or after the tour. We never did cross over the Galata bridge to the new part of the city in 5 full days there. We did take the ferry up to see the Black Sea and we spent only one day on the Asian side.
The museums, Topkapi palace, and the mosques were fascinating. We also took the Istanbul Eats tour of the Old Town. Worth every penny. That gave us our bearings and the confidence to travel to the back streets to eat and shop where the Turks live, eat, and shop. BTW, the Turks were the friendliest, most helpful people we have ever experienced and we've traveled world-wide. Don't short change yourself in Istanbul.