As a higher level player, I can say that I would rather use my wish for an island since they are so expensive the more you have. While it is true that they are available all year in the regular market, if there is something else I want to use my wish for (which currently I have none since I used my last one on an island) everything is available year round in the wish market so I can just wait for a new wish to roll around. Since wishes come around every 100 days of logging in, it's fine to use the wish on whatever you want. Back when wishes were first introduced, I always saved my wishes when I got them. I think it was about a year or so before I used my very first wish.
I'd say if you have the wish and need the space of a new island but don't have the dragoncash for it, go ahead and get the island. Wishes may take a while to earn, but they'll come around again.
Celt definitely makes a great point! It really depends on which island you want to spend your wish on. If it is one of the earlier islands, it might be quicker to save up the Dragoncash than to wait for a Wish. Similarly, seeing as Wishes are granted every 100 days, it might be a lot more convenient to utilize a Wish on a more expensive island than to grind daily in order to collect the required amount of Dragoncash.