a mobile sandy bridge core i5 would deliver superior cpu performance (compared to a mobile i3) with aes-ni, higher clock speed, turbo boost.
For regular office applications like "Microsoft Office - Excel, Word and Powerpoint... etc" most if not all i3 will be sufficient. However, if you prefer the system to run smoother and cut some waiting time on larger powerpoint files (>20MB in size), I would suggest getting i5. Asus Zenbook UX31E-DH52B 13.3" Ultrabook ~Expensive, yet stylish Samsung 300V3A-S02DE 13.3" ~Price in between and functions just as well Fujitsu LifeBook SH531 ~Cheap alternative, that is bulkier
More often than not I tend to find that more RAM is better than more CPU for Office tools; many people have many opened applications that use RAM, but usually have only one "active" at a time (unless you run lots of background jobs like music/video processing).