Evolve good iv chansey or high cp chansey?

I think that is a tough call. You could immediately make a strong defender evolving the lvl 29. A strong Blissey you could use here and now before Blissey is possibly nerfed or a counter to Blissey is introduced.

On the other hand, just the thought of a 96 IV Blissey is awesome, but it is gonna require a hell of a lot of walking and/or lucky hatches to get it up to speed.

Honestly, I would go for the 96 IV, but I think either way is solid. Conceivably you could evolve the lvl 69 and then not power it up further while saving up for a future evolvement of the 96 IV.

I personally would buddy up with Chansey until I had a lot more Candy to instantly power up the lvl20 after evolving...perhaps another 50+, and hope I hatched more as I walked.
Then evolve the higher IV but lower level one.

If you are desperate for a good defender right now, then that might change the answer, as the lvl29 Blissey will be of use immediately.

Off the top of my head, there will be around 250CP difference between these 2 at level 30 - that can make a massive difference in gym position.

For CP at level 30.

15/15/15 2759CP
14/15/15 2756CP
15/14/15 2740CP
15/15/14 2753CP

So, in conlusion, at level 30 for Blissey
1 IV to Attack is worth 19CP
1 IV to Defence is worth 6CP
1 IV to Stamina is worth 2-3CP

It happened to me... And I choose #2. She can stay longer in a GYM.... And I did not regret because that Nurse help me collect a coins without using a revive...