I would go to a shop and have a play, however I wouldn't get a "mini" version if your wanting a good spec phone, there good still of course but lack one or two features that a normal HTC ONE or S4 would have. And I think you mean iphone 5S? The 6 would be next year.

Either of those 3 would have the ability to tether.

If going for the iphone make sure it's the lastest 5S, the 5 did not support all 4G/LTE bands, only a few were supported, but with the 5S there's more bands supported.

Depends what you mean by using as a portable HD? You can put pictures/vids/music on, if you mean like plugging it in via USB and dragging/dropping files then it's not quite as easy as that.