for TTL you really want a canon, but the Nissin will give you as good results; might need some RAW tweaking but not the end of the world.
My advice would be go to jessops / camera world / any camera store try both the canon and nissin units, compare the results at home and then choose.
What difference does it make for multiples?
consistency, if you're using the same flash units then it's easier to balance the them e.g. 1/2 power on a Nissan wont be the same as 1/2 power on a sigma.
Ideally you'd want them all the same that way you've got a backup for on camera if your main one fail, but hey on here we're not made of money.