i'd rather get a PM in response to an ad, than an unsolicited PM offering to "train" me!
A forum post, in response to an ad, would get less of my attention than a PM. So PMs are better in that situation, and give the Dom an opportunity to say more about themselves than they might in the forum post.

but hey - why not both! PM me, then reply to the ad saying "left you a PM" - kind of like the wolf peeing on the gate post, marking his territory.