I can tell you one thing, you wont be able to ruggedize an iPad any more than any other brand. you want your climbing boots to be durable but electronics are prone to failure if you drop or misuse the hardware so it may be worth you getting a support contract from a hardware supplier to replace/repair them when they break.
I don't see why you just don't get on of these http://www.asus.com/Eee/Eee_Pad/Eee_Slate_EP121/#specifications
- it will run your app with no modifications
- if you use Windows 8 you will need to rewrite it in WinRT, what do you gain?
- How much will it cost to develop and when will the iOS version ship, while you are talking yourself out of a job promoting a platform you cannot code in (yet)
- Android phones have a remote wipe facility that sys admins tend to like, not sure if iOS or WinRT/Metro will have this
- WinRT/Metro will ship in October/November time or just before, even though there is a go live license, I would never develop software with a beta product (ever)
- iPad is proven, tried and tested, WinRT/Metro has a potential to flop as no-one knows of the gamble will pay off yet