best way to spend Chaney candys

It depends on how you use the mons and how competitive your gyms are. If you try to have 10 mons in gyms at all times, then it may make sense to start work on another defender now. I never get more than 20 coins per day due to very high gym turnover, so I just focus on one Blissey.

For Blissey to be most effective, she should sit somewhere in the middle of the gym. In my area, that in itself requires a lot of candy, but I want to have a Blissey to put in gyms, now. In my situation, I would focus on maxing out the one you have.
Conversely, Blisseys are big investments. If you don't want to use one, now, you could work getting a perfect one to invest in once you have it. That might turn into a long term project, and things may change in the meantime, or you might be happy with the result in the long term.