I have never been south of Europe yet.
For my next trip, I would like to visit either Marrakech or Istanbul at the end of October, begining of November.
I just can't decide!
Eventually, I will visit both locations, but for now which one should I see first?
I will be able to spend about 2 weeks there. So I will want to take some day trips, or move from town to town.
I also would like to know how many days I need to visit either town.

I need your advice.
What would you recommend?
Thanks for your help.

I have been to Istanbul, it can be enjoyed in 2-3 days. The location is entirley amazing, and there are other destinations. It is a much larger city than Marrakech. Clearly, both are fascinating.

Both cities are absolutely worth a couple of days, not two entire weeks though. I'd say you will need a week at most. Depends a bit, how intensively you want to explore.
From both, you can go on with your travels. Lots of things to do.
Can't help you decide. It would also depend how easily and at how much I could get there. In general, Marrakech is a bit more difficult and might involve a plane change. But all depends where you're flying from.
Honestly, you can't go wrong with either. But, please do some homework and get a good travel guide book to avoid a culture shock which might be more significant, if you decide for Marrakech.

Have fun whatever you decide!

Both are great destinations, but for a two week trip you can do more that one city in either country.

As Ursula mentioned alot can depend on from where you're travelling. It's about 6-hr to Morocco from NY; whereas Turkey is about 9-10 hrs. Also to consider, is the weather at that time of the year.

End-October/begin-November is starting to get cold in Turkey, if not already; whereas, Morocco will still be warm and probably a more ideal time to travel there.

Get a few guidebooks and search on the web to find suggested itineraries for each country to give you an idea of what many tourists wish to visit/see.

In both countries you can travel on your own (driving) though Turkey is a bit more trecherous as it's also more crowded and a much larger country. Many people visit Turkey in the areas west of the middle - Ankara/Cappadoccia west to the Aegean. Morocco, on the other hand is from north to south and west to the Sahara.

After you've pinpointed your interests in each country I'm sure you can get some more information from this board.
Happy research!