I would really suggest not staying at the Ren if you want to see Istanbul - it's a long way out and if you want to go back to your room mid-afternoon before heading out again for dinner, that's going to be a lot of wasted time.

I stayed at the Hyatt a few years back and thoroughly enjoyed it. If you do this, make sure you sign up for Gold Passport - even at the basic level, we got free tea or coffee delivered to the room each morning at whatever time we asked for it!