I was in the exact same situation and drove myself to a migraine. I just went to the Apple Store and spent some time playing on both devices. Granted the iPad Mini with Retina is not in the store, but the original iPad Mini is. The weight/size is nearly identical so I used that as a testing object. I spent a good hour+ moving from table to table testing popular sites that I visit, games that I play, reading newspapers, and watching YouTube.
I found that after 20 continuous minutes of holding the Air my hand got "heavy" and I started to switch back and forth between hands. Granted the screen size was nice, but I know that if I am on a train/subway that I will be holding my iPad and not using a smart cover (or alternative stand). Therefore, holding it was important for me.
When I was doing the holding test I did take a break between the two deceives to give my hand/arms a rest. During this time I looked at the accessories and tested out the Smart Covers and Cases that they had for demo. At first I went in with the intentions to get a Smart Case, but after trying it out and feeling the leather material I deemed that I did not like it. It functioned great; I just didnt like the material. I actually like the Poly-whatever Smart Cover. I decided to get one of those with a clear back cover. I dont like colored cases because the finish of the iProducts are really nice and I dont want to hide it. I decided to go with a green Smart Cover.
Also, I found myself getting annoyed by not being able to type using my thumbs on the Air. The most typing that I do is sending an email, typing in web address, or creating grocery list. I personally do not like being required to stand my iPad up just to type. Maybe I am stupid but I dont see how people type on the iPad like it was a real desktop/laptop without the iPad being supported by something. So, I do not like to test typing that way. I like to hold in with both hands and use it. With the Air the only way I could type was to poke at the screen with 1 finger, because the other hand was holding the device (which again was heavy) and my thumbs could not reach all the keys (in landscape or portrait mode). This may not be an issue with those with larger hands, but as a girl with smaller hands it was really difficult and annoying. Blame me for being used to typing with my thumbs on my iPhone, but whatever. With the Mini I did not have a typing issue. I was able to type quickly without any typos or extreme stretches of my thumb (in both orientations) and I didnt get tired of holding it.
The two different screen sizes didnt bother me. After watching movies on my phone for over a year, the larger Mini was good enough for me. So I made the decision to go with the Mini. It is also $100 cheaper with the same insides as the Air. That is $100 I can use towards Apple Care+ or accessories. My best advice to you is to just go to the Apple Store and play.
Never! 
