If I convert an AVCHD file from my Canon HFS10 directly via HDLink vs exporting as Cineform AVI from Premiere Pro CS4 is there any difference?

There are times I only want to work with a part of a long clip, and using CS4 would allow me to trim the clip just to the part I want to use and use less disk space. Will I get the same quality?

Close enough. There are advantages to using HDLink for quality, but you may not generally see the difference.