I am attempting to pull posts from my Wordpress (on the same server as my main site) to my main site.
I am able to use both RSS and Ajax - but I need to choose between them.
What are the performance pro's and con's of either, and what should my final decision be?
You can choose to use RSS, then you can choose to use it via an AJAX request or not. AJAX itself is not a mechanism to pull the posts. What exactly is the other option available to you?
I'm going to go out on a limb and say you mean (by RSS) that the feed is to be parsed and served inline on the server side.
If that's indeed the case, go with RSS. You'll need to implement some sort of caching (to keep from hitting your RSS feed over and over and over) but the speed benefits will be noticeable.