Hi David,
I’m sure you know, but these are two different cities. Mianyang is actually a pretty large city – second largest in Sichuan Province after Chengdu, not counting Chongqing of course. It has a solid reputation as a good place to live. Pretty clean, friendly people, etc. It’s not too far from Chengdu, so visiting Chengdu on a weekend would be reasonable.
Jiangyou is farther removed. If you already speak Chinese well or if you’re interested in less foreign influence, Jiangyou might be better. If you have a choice between the two, I would choose Mianyang, since there are a handful of foreigners there already which is nice to have. I’m sure there are some in Jiangyou as well but their numbers must be very small.
Out of curiosity, why do you want to leave Hainan?