If it doesn't matter to you the stealth value, then go with the Solo. It has 2 extra tempretures that you don't get with the Air. I have both but I use my Solo way more than my Air.
The build quality is probably the same since they are from the same company. The Solo has been out since 2011. The AA came out 2 years ago last Nov. You don't have the user replaceable batteries but the Solo lasts for 2+ hours when new. The Solo's battery can be replaced after 2 or 3 years whenever you feel the time is right. You can buy a battery for around $40 -$50 dollars.
I've dropped my Solo a few times. Other than a chip on the bottom plate no other problems. The bottom plate on the Solo has been an issue. I ended up buying a stand for mine from
@Pipes a fellow FCer. It covers the bottom plate and it works really great.
My $ is on the Solo. I prefer mostly an all glass air path unless I'm using wood so the Solo and the Air fit my portable needs to a T.
Here's mine with the
@Ed's TnT mouthpiece and the stand from
@Pipes. I had just finished doing some gardening when I took this pic.