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/Peo, FreeWebSpace.net
Well, MYSQL 5 gives you more security advantages, and usually the more updated version wins. You should really address this question to A2hosting.com support.
What type of site are you planning to run on their server?
I think it's better to use a proved version of MySQL. The new one is unknow and has finised yet. I don't see any reasons to use it. Though A2hosting.com provides both versions, I'd stick to the old one.