Apple only sell one latest AC model retail, although doubtless there will be plenty of the old one cheap in the refurbished shop for the next few months.
what is they best time capsule for my air mc book.
I am unsure what the question means.
If you have a new MBA that is AC certainly the latest AC model is a must. For the older model with N wireless you can take your pick.. but don't pay too much for the older one. I think it is still best to buy the latest although some people have found it doesn't work as well as Gen4 TC.. others find it works much better.. As with all things wireless it is 50% science and 80% voodoo (in voodoo it doesn't even have to add up to 100%)
PC laptop can use TC for wireless and can store files on the hard disk, although the later can be less than super.. it might be better to buy a 2TB version.. and buy another 2TB USB drive and plug that in.
Mixing data and backups on the internal drive.. still the same in the latest version .. there is no access to the disk to partition or do anything serious..
See Pondini. Q3 http://pondini.org/TM/Time_Capsule.html