Definitely the infuse. I've hard a lot if issue with the atrix. Like the battery life and it start randomly rebooting.. infuse is ok so far. Better battery life, bigger screen, super amoled screen. and a much faster dual core.
the Infuse is a single core, and being a user of the Atrix, Inspire, and Infuse (yes I work for at&t) the Atrix has much better battery life, however the screen on the Infuse is 1.5 inches bigger, and the Super Amoled Plus display makes everything look fantastic. Although the Infuse is bigger its so light I barely notice a difference in physical size. Playing games on the Infuse is better because of the screen. However the Atrix is better built, and gets better signal in poor area. I was never a fan of motoblur, so my answer might be a little biased, but I feel the Infuse is a more entertaining and overall fun derivce then the Atrix.