Susan,
Seems like you already got tons of recommendations, but noone mentioned another important issue here. Getting small flash is not always a good idea. I have an Aria and have been contemplating getting the small flash as well. After much thinking, I decided not to. Why? First of all, because in all situations where small flash won't do - I will need a bigger flash. Which would mean - I would need to have two flash units. That means - more money out the door and still, when I take with me the small flash and not the big flash it would mean that if I run into situations where bigger flash would have been more useful - I will be sorry I didn't carry along the big flashhead instead. So, eventually I may end up carrying BOTH units, which makes little practical sense. Then, I would start carrying only the big one and the small one will collect dust. Thus, what's the point?
Now, what small flash often doesn't give you? Fast recharge times, tilting and rotating head, various bounce adapters, flexible and easy to use flash power adjustments and more control modes (e.g. Auto, TTL, full-manual, reduced power settings, etc), secondary head (often with light-reduction filter), higher light-source point (no red eyes, guaranteed, less direct light into eyes of your subjects as well - thus less annoying them).
So, I have Metz 54MZ-3. It has tilting and rotating head, secondary flash head with "switchable" light-reduction filter, different TTL modules (I also had pentax system for a long time and canon for a short time, so it was useful that I didn't have to buy new flash for each of these) and I also have bounce-adapter card for it that I use most of the time. It doesn't balance too well on Aria, but considering Aria's design - almost any decent flash won't balance well. But it's something that takes getting used to, after that it's okay.
Only really small and light flashes will work okay on Aria as far as weight balance is concerned, but it will be way too restrictive in my opinion. Considering that flash photography is fairly complex and takes a lot of practice to do well, getting some dumb and small flash might be a bad idea.
That's just my 2 cents.
Mike.