iOS 5 works well on the iPad 1, and as it has just come out I wouldn't worry about longevity too much.
For anyone submersed in the Apple ecosystem, particularly with lots of apps, using iCloud etc, the iPad 1 would be the easy choice here. It's worth remembering that, while the competition is improving, no one has really 'cracked' the iPad yet. Smartphones are a little different, manufacturers have taken the iPhone design and started to break away, but for tablet computing the iPad really is where it's at - and yes, I mean any iPad.
Similarly, anyone in the Amazon ecosystem may well find the Fire a natural extension of that and should probably go that way.
I still think the original iPad will prove the better tablet.