The Pegasos 1 and the A1 is quite similar when it comes to hardware. However, the Pegasos is smaller, it has better features (Firewire and an optical S/PDIF digital audio out connector), and is **a lot cheaper**.
If you look around you might find both the Pegasos 1 (with April2) and A1 motherboards available right now at dealers, for instance I believe that
GGS Data in Sweden has managed to get a few of them both in stock.
If you choose the A1, you will have to run Linux until OS4 is ready enough to be released, which probably won't take more than a few months now. If you chose the Pegasos, you can start using it as an "Amiga" right now (with MorphOS), as well as a lot of other OS's.
In a (few) week(s) or so, the Pegasos II will be for sale. That one is an improved Pegasos 1, with a new, very powerful NorthBridge, DDR memory, 1 Gigabit Ethernet and 1 100/10 mbps Ethernet connectors, etc. When the Pegasos II goes for sale, there will be some second hand Pegasos 1 for sale at about €99 (?).
In a more distant future (speaking of the first quarter of 2004), a mini ITX version of the A1 is expected, as well as the Pegasos III, which will be a G5 computer!