For starters they both use AMD hardware and ATI graphics, rather than the very common Intel hardware and Atom processors.
The Acer Aspire One 521 has a 10.1-inch LCD with a standard 1024 x 600 pixel resolution, while the Acer Aspire One 721 is the larger at 11.6-inches with a 1366 x 768 pixel resolution.
Both make use of the AMD Athlon II Neo K125 1.7GHz processor and ATI Radeon HD 4225 graphics which give both machines HD playback capabilities and more power than an average Atom powered netbook.
Although offering more horse power, it does have an impact on battery life, with the with the Acer AO521 lasting 4:02 hours, and the Acer AO721 3:30 hours. While not great, it is better than the previous AMD offerings.
This puts these netbooks between a normal Atom powered netbook and an CULV-powered notebooks. While they might not provide as much run time as a CULV notebook, they also have a much lower price tag of $350 and $430, where as a CULV notebook normally starts at $700+.
Both the Acer 721 and 521 were reviewed by Engadget. They were impressed with their thin chassis, performance, HD playback, and low pricing. But were disappointed with the battery life and flat keyboard.