Renaissance is closer to the Nezavismosti street. We walked from the hotel to the square in 20 minutes and from there continued until the DoubleTree hotel and back,so it is walkable. But that was in the summer. We had a car for use,so did not used taxi or public transport. From Marriott to city center is approx.3 km,maybe 4-5 bus stops do not no exactly.
I personally and my friends would choose Marriott over Renaissance for the following reasons: We always had a suite upgrade as Platinum,executive lounge is bigger and food selection is wider,breakfast selection in the restaurant and seating space is wider. But the absolutely best benefit was a gift voucher to use Falcon gym(could join training class,we did TRX) and SPA. But Renaissance is a solid hotel and I would definately stay there again,no doubts. Of course now is new year and there might be some changes to gift voucher benefit.