That said, Smalls and up on EC2 will give you more consistent performance (they aren't built to provide the spike-performance Micros are) BUT are still relatively subject to "noisy neighbors".
The smaller the instance, the bigger the impact felt; it is still possible to have a small grind to a halt because of a noisy neighbor and typically I do not see people hosting web front ends with smalls because of the erratic and poor performance (in general). Possibly a fleet of them behind an ELB, but even then I see Mediums and Larges much more often as the "web server".
The mediums provide an awesome (and typically unsung) balance between monthly cost and performance on your way to a large (which can get quite expensive).