Hi Marky,
To be honest, the answer to your question is that the "$700" buys you absolutely nothing. OS4 might be lighter (and thereby a little quicker feeling) than Windows or Linux but the speed difference will still be tangibly slower on the AmigaOne than a PC.
I cannot really state any benchmarks on the AmigaOne. I've never seen one. I've also never seen OS4.0. I'm speaking strictly on the facts of physical hardware speed.
That being said, I have seen and played with, first hand MorphOS and the Pegasos (mine is not fully assembled yet due to money issues). Compared to any Amiga that I have EVER seen on this planet, the Peg/MOS combo is truly phenomenal in terms of the "feel of speed" and ease of use. Do I feel that it's faster than the PC? Sure, in a lot of respects I do. It feels lighter, is definitely more logical than Windows and the Pegasos/MorphOS combo really brings back the feeling of fun to tinker with. I have no reasons to believe that OS 4.0 would be different. Now... Is it really faster than my 1.4 Ghz Athlon PC? No. Not by a long shot. No Amiga has physically been faster than a PC since about... oh, I don't know... 1992?
You said it before though, a lot of the reasons for an Amiga platform system are based on Nostalgia, not power. The ability to have FUN with a computer. The ability to learn and know what the OS is doing in the background. The lightweight OS with a logical look and feel. The quick boot times and no "shutdown". There is also (or at least there used to be) the tangible, friendly, helpful community surrounding it.
In the end, the choice for you is exactly that, and it's a very personal one. What I would do if I were you is to buy what you need in order to meet the way you want to use a computer. If that happens to be an Amiga platform machine, very cool. If not, no harm, no foul. No one (except the zealots around us) could ever slight you for exercising personal choices in computing. An Amiga user chastising someone for using their personal freedom for computing would be somewhat of an oxymoron I think.