The Celeron is a single core, single process CPU and according to Intel that Atom is a single core, dual process CPU (it has hyperthreading) ... however, that along with Atom's clock speed advantage still isn't enough to overcome that Celeron. If you want to know which is fastest, the Celeron. If you want to know which uses the least power and puts out the least heat? The Atom. Unless ultra long battery life is an absolute must, there are better alternatives out there for the uses you mentioned. You will be much, much happier with a lower-tier dual-core Pentium laptop than either of those two suggestions.
Wow. That's a big pill to swallow. Just how much are these laptops? If at all possible consider a 15 inch laptop... yeah, it's a little bigger/heavier... but you won't be paying a price premium like you are for that ultra-small laptop. If you absolutely demand something that small, I'd say it depends on how much extra $$$ you have lying around... the Celeron is faster... but man, at that cost? It's a tough call.