Vincent Stemen did a good amount of testing of cvsup vs. rsync in terms of update speed. Rsync came out way ahead, though as a few people noted, rsyncd’s load relative to cvsupd on a server serving many clients is unknown. In any case, cvsup does not build on DragonFly via pkgsrc, so it looks like rsync is generally a better choice, plus most mirrors are using it now. HAMMER may provide a better alternative, in any case.
While on the topic: Ulf Lilleengen’s blog post about improving csup.