Concerning cross-promotion: it kind of has its ups and downs. On one hand, if you've got a big comic with tons of ad revenue, it might help keep the 'supply' of links low and their value high. There aren't *too* many comics like this, though - sore thumbs, sinfest, and QC, I suppose, make enough money to actually care about this.
However, if you aren't horribly concerned about a potential difference in ad income, cross-promoting is the way to go. Cross-promotion, by getting the names of many comics out to many readers, is more likely to cause people to read the comics of the highest quality that interest them most. If you are confident in your comic, cross-promoting is the way to go. I've gotten only positive results so far from linking to and being linked from my peers in my news posts.