Best of Spain or Best of Turkey

I've not been on either but I think a lot of it depends on not just your likes/dislikes but how much you've traveled, your fitness level, your age.

For myself, I'm mid-70's so would choose to do what I perceive as a "harder" trip first - which for me would be Turkey. It's a bit harder to get to for me from the Inland Northwest. A bit more out of my comfort zone culturally and food wise. However, I can see the light at the end of my travel tunnel and am wanting to get trips done that I might not be able to do in 2 or 3 years. You may be much younger than I and have plenty of travel time left!

The Activity Level on the tour website indicates Turkey is strenuous and Spain is moderate to strenuous some days.

IF someone is new to travel and this is their first trip, I might go with Spain because it seems more easily handled.

Any chance you can do both of them back to back? I've done that before and it's a great way to take advantage of just doing one long flight to Europe, lol!!

I've done both tours - Spain in spring time and Turkey in late September. Both great. Different experiences though, so I suppose it depends on your interests, fitness level, availability of amenable flights, etc. If you're really torn, draw straws and plan the other for the next time.

I traveled (as a teacher, in July) on both tours although the Spain tour was the "My Way" version. I loved both! And it will be an ideal time for either with (presumably!) less crowds and pleasant weather.
You have received some great input whereas I still kind of hesitate since it's just so very subjective. I felt like I was able to gain a cultural appreciation both times with the RS tours, beyond just checking "must sees" off a list - while still seeing the must sees!
Both had a lot of walking, and maybe the ground was uneven or cobbled in places, but I didn't think it was at all physically challenging (no hikes, etc). OTOH, maybe the BoS tour has hiking options?
If either or both of you has any mobility challenges the detailed itinerary clearly describes what you're likely to encounter, as another way to decide
If you do choose Turkey the hot air balloon ride was phenomenal. It's so expensive and you get up so early but absolutely a must (and I'm horribly afraid of heights).
Spain seems like the "safer" option whereas Turkey is different enough that it adds such another dimension. Also, for ancient history (fascinating!) I'd say Turkey? Which almost segues into Greece and combining those two tours would complement each other- or- add on your own before or after.
How to decide!? But you're going to have an amazing adventure regardless!