What's better, one large tank (180g) or two medium tanks (90g) ?

[kien]

So I know that I either want to upgrade my current 90 gallon mixed reef to a 180 gallon and close down the 90 gallon, or set up a new 90 gallon sps coral only tank. There are lots of pros and cons for going either way and I think I have thought out most of them, but I figured maybe some open discussion and other opinions would be good.

My personal reasonings have broken it down to two main points.. I like the idea of having two tanks in case one fails. Kind of like a backup. The old don't keep all your eggs in one basket cliche. Over the past week I have ready through tonnes of build threads and have seen some horror stories.. I suppose it is possible that both tanks will fail but hopefully not at the same time! I don't like two tanks because then I have to maintain them both and there is a lot of duplication, potentially costlier, more maintenance ? Anyway, I am conflicted. Thoughts?