There are some similarity after a few years jumping into web design and developing. I find that Ajax has been widely use in the recently, due to the fast loading and simple effect that it can provide to the end users. Anyway, all of them could give some interactivity with the end users, but this is about how good they can be.
Ajax beat Flash hands down. Fast loading is what the internet users are looking for nowadays. Unless your Flash has something “Whoa!” or it’s just a waste of time loading it to the impatient end users. While I was about to get into Flash, now that Ajax has strongly attracted me after a few year. I choose Ajax instead 
No doubt, I want something simple yet fast.
Flash = Game, Ajax = Site. Nowadays the clearest differentiate between Ajax and Flash. Fully flash site is so yesterday already, though it load the whole pages and links after a while, but it doesn’t mean Ajax can’t do it, check out onepagelove.com and you’ll see an impressive list of one page Ajax enable sites 
Freelance web designer would have to recalculate their quotation now. Because there might be a client require to design a page. Well, unless the freelancer still using hourly based quotation. Time to get some area rugs and sit back for Ajax learning dude. Even in my office, all web designer are taking their time implementing Ajax into site for interactivity 