Nintendo does have unified architecture and to be honest I wouldn't be surprised if they kept it that way. It would be sort of cool if they made it backwards compatible to DS, but I think that might be a bit too much, but be like the first console to go backwards compatibility on 2 generations than just 1 generation and the rest Virtual Console.
I don't know about AMD because Nintendo have never used them before and they use ARM chips. I think it all depends on price and what it can provide and bring to their next-gen portable. I think they might wanna stick to Vita's Power, but maybe a bit more of a kick. They learned their lesson that it needs to be somewhat cheaper than the launch price of the 3DS. I have full faith they will do good with the next handheld.
