For the most part unless there's a significant level difference, you should go for the higher IV's, in this case they both are level 20 so that's not a factor. However, for Blissey the attack stat is the one that will have the biggest impact in her performance, as her defense and hp are already pretty high. However, in this particular case, the difference in CP is going to be 5 CP at level 30, it's so insignificant, you have to give the edge to the 88 IV one.
GamePress
Which Chansey to evolve?
I finally have enough candies to get a Blissey, but I'm not sure if I should evolve one with higher IVs or higher CP. Here are the two Chanseys I have:
lvl 20, 826CP, 13hp/15atk/10def
lvl 20, 820CP, 14hp/14atk/12def
The first one only has slightly higher CP, but that can make a difference in gym placement. Any thoughts?