the Atom's aren't designed for speed so much as size and TDP. they'll be in teh smaller (7") notebooks because of their low heat signature and size. they perform like they should, using like 5w of power. they won't play games well, but will play dvds, and will struggle with some stuff. you have to tailor your software to make sure it'll take use of hyperthreading (assuming your model will have it, HT increases cpu power up 37% or so.)
The celeron (assuming it's the C2D variant with frequency and cache deficits) is the faster choice, rarely losing in benchmarks (<1/10) to the Atom, but also uses more power, and if you want a small notebook, i'd go Atom, if you don't want a 7" notebook, go celeron (or celeron m, pentium 4, c2d, c2q, w/e).
http://www.tomshardware.com/reviews/Intel-Atom-Efficient,1981.html