I am almost finished my first HAED and have used 18-count aida. Previously I'd never worked on anything smaller than 16, but bought a kit with 18 and it looked really small to me.
The overall project is pretty big, but I've felt that for all the work that goes into the HAED designs it should be! I haven't lost any of the detail or anything.
I have had to buy a couple more skeins of thread though, as I have run out of the colours that are used the most. But it hasn't been too bad.
The only downside is that up close the stitches are very big - although when it is hanging on a wall, the further you stand away from it the better it looks. (this may be the case with every count though). I am contemplating 25-count for my next project, only because it is even bigger and will be about the same size as this one even at 25 count.
Our Micheal's is mostly useless, although I did stop in at an outlet-mall Michael's this weekend, and they had all of the aidas and one 28-count monaco fabric (it was scary looking. I am going to wait until I can see a 25-count piece up close and personal before I decide). Go with what you are most comfortable with, the worst case is that you might have to grab a bit more thread later on.
Here is a picture of my 18-count Aida HAED:http://i492.photobucket.com/albums/rr287/heathermarierose/DSCN1727.jpg