Like others said, iPad 1 and iPhone 4 aren't too far behind the newer models.
I am one of "those guys" that upgraded both from the originals. I can tell you from experience that I like the ipad2 upgrade more than the iPhone 4s upgrade.
I use the iPad2 a lot for pdf books/magazines, and its very snappier side by side.
The iPhone 4s is snappy as well, but I think the main awesome point of it (for me) is IOS 5, which you can get on the 4 anyways. Im not a big picture/video taker, nor have I found a way for siri to come into my everyday life.
So if you really want to upgrade one of them. I would upgrade the ipad2, but honestly like others said, both older gens are still really awesome