How would you rate the quality of life in Danang?

Compared to my home country:

Obviously more affordable and I enjoy the change in culture, the beach, the scenery and I find the people friendly.

Compared to Nha Trang

I lived in the tourist area in NT and everything was nearby. I am now in the beach area of Danang near Holiday Beach and find it not as convenient. Not as many mini-marts, no atm's, pharmacies, print shops, bank is farther, etc. The beach is better for swimming and there are more interesting places, e.g., World Heritage sites, historical sites, etc. Both have nice walking paths along the beach.

Accommodation is similar. Comparable apartments can be found in the same range of prices whatever your budget but looking around is a good idea as some offerings are considerably overpriced. If planning a long stay, it might be a good idea to stay in a hotel for a week or so and walk around checking apartments rather than renting online.

Nha Trang is supposed to be bigger tourist area especially catering to Russians. 

I also hear the locals there are not quite as friendly because of that.  Any truth to this?

Actually Nha Trang is now overrun with Chinese.

Yes I think maybe more Chinese than Russians now. I don't know where you heard the locals there were not friendly. I found most everyone there quite friendly.