1 1/8 is stiffer/stronger, and currently the industry standard. i don't really notice a difference but then again i haven't done an exhaustive double blind experimental procedure to care to find out.

you'll mainly see 1" threadless on older/steel/ti frames as steel frames use smaller diameter tubing.

v20 offers a nice, clean 1" threadless stem. i like threadless better than quill, anyway (flamesuit on).