I drive a 500 TA. Great little motor!
It pops along when you want it, and can be economical too when you want it.
I broke my personal best the other day on my commute home, 71mpg over 14 miles.
If you go to the honest John web site and look at the 'real mpg' section, real end user entered data gives a very good indication of actual mpg's people are averaging. On the 500, the 1.2 and TA are nigh on the same around 49 mpg.
I wouldn't be surprised if there wasn't much 2012 panda recordings in there at the moment, but I'm sure the panda wouldn't be too far off 500 figures??
You will not recover the extra you paid for the TA over the 1.2 in fuel bills, but you will have more fun I reckon driving it and you will get more when you sell it eventually.
Oh, and forgot to say, in case you didn't know already, the car will feel very tight, it wont like low revs, and your mpg will be a lot lower when brand new. I'm near 4k miles now and the Mpg is still climbing, and the motor is losening up nicely.
You've made a good choice, enjoy when it gets here!