Regarding the Ren Zhongshan Park, I hear you on the positives. However, as first-timers, non-Chinese speakers, who will be challenged enough dealing with the basics like AliPay, and trying to figure out what (never mind where) to eat, we likely won't get much benefit from the Ren's location. We probably wouldn't figure out which are the cool food and drink places if it hit us in the face!
I can appreciate how the location might be unbeatable for those who are already familiar with the city, but for those like us who just want to step out and walk to the Bund, Nanjing E Rd and maybe visit the French Concession and try a couple of famous restaurants, the closer to the Peace Hotel the better.
So I think I'll stick with the Marriott. The food at the Ren lounge looks awesome though!