There isnt as much support for it yet. If I recall the last update of Final Cut Pro started supporting the format. From what I know about AVCHD it has to be transcoded and its really processor intensive so it will probably require a powerful machine to do anything in real time. If its for home stuff Id suggest HDV since its supported in so many more applications and youre more likely to find more help with your workflow since its more widespread.
Then again if youve got the chops to try something new go for it.