The Acer Aspire One 'Netbook and it's single core Athlon II Neo K125 is just a slight bump up in power over the Atom based Netbooks.
The Acer Netbook will have longer battery life and a smaller 11.6", lighter form factor.
The HP G42 has that substantially more powerful dual core Athlon II P340, a bit better GPU and 3GB RAM and a bigger HDD.
A bigger 14" screen, a bit more weight and shorter battery life are the flip side of the more powerful laptop.
The HP is a better all-round computer, and it's a bit less expensive to boot.