Well, its really up to you. The Thinkpad's are usually for business, but they are $100 cheaper than the lowest end Macbook Air. The Thinkpad also looks higher spec'd than the Macbook Air, but thats understandable, they're Windows based computers anyway.
I have a Macbook Air, and I totally love it, so of course I would recommend an Air

, but to give you a realistic look...
What OS do you really want?
The Macbook will allow you to run both OS X and Windows (via Boot Camp), but if you need to use Windows and only Windows, get the Thinkpad.
Do you like style?
Macbook Air obviously wins here, but the Thinkpad's are pretty solid business machines, I see them a lot at school.
How much do specs really matter?
I think OS X's experience is much better than the Thinkpad, so while the Macbook Air's specs are lower than the thinkpad, it still runs very fast for what its meant to do (now pretty much everything with the Sandy Bridge chips) The thinkpad obviously has the higher specs, so that may be something you want to consider as well.
The thinkpad would make a great work machine for Windows, but after using a macbook air for about a month, its more than capable of doing work both in OS X and Windows. The only thing I'm worrying about is compatibility since I don't use Office in OS X. I am curious as to know what you will use the machine for. You said you had an iPad, a Macbook Air would be a natural upgrade, but do remember that OS X isn't very much of a gaming platform.
If your going to use Windows based applications for work, then its probably best to get that Thinkpad. They're pretty well known for being professional work machines, they also have that track pointer and fingerprint scanner. Those are cool features I think are nice to have, but I haven't really experienced those features myself.
So thats my take on it.