I built up a touring bike several months ago and was going to install v brakes but my good friend and shop mechanic said I need to use the Shimano Cantis so the brakes don't interfere w/ my rack but also for better brake modulation. I tried the brakes for about 3 weeks and exchanged them for v brakes. The modulation argument doesn't hold any water at least for me. Get my 225 frame on my fully loaded bike and there is more modulation that I ever could want. Even if its not loaded. Also, I had a hard time stopping w/ the Shimano Cantis. They were set up correctly and double checked by the above mechanic but the stopping power was not there. Finally, I couldn't obtain equal lever resistance for front and rear. I/we tried every conceivable combination of spacer etc... but the rear was way to soft. Anyway, I am glad I tried them again because it reminded me how much I dislike them. Last thing I want to sound like is a know it all and I realize they must have value because there still plenty out there and people like them. I need to add that I just built a bike for my girlfriend and it is a small frame so installing the rear rack w/ v-brakes was a challenge. Fortunatley I had a Tubus rack w/ bendable struts which made it doable.
I used Avid Single Digit 5s on my bike and Deore LX on hers and they are both great. I would save a few dolars and get the LXs on my next bike.