Thanks for all the advice guys, I would normally go for the manufacturers design but I don't quite get why Honda are selling it as an accessory and not fitting it as standard, unless they only think its required for off road use.
The Powerbronze unit seems to be most popular so I think that's my preferred option.
I've only had the bike a few weeks and only used it on roads but there is a significant amount of dirt thrown over the shock so even if its not an engineering essential it makes the bike look crap without it.