I'm not a fan of low mileage older cars; they tend to be expensive for what they are and can often cost unsuspecting owners thousands in repairs over the next couple of years. In the UK climate, age, not mileage, does most of the damage.
Personally I'd sooner buy a properly maintained '14 plate car with 70k than an '11 plate car with 13k, and you'd likely get the first car cheaper than the second.
I wouldn't go so far as to say that there have been a lot of problems with the TA engine, but there have been some and if you are unfortunate enough to be one of the some, it could prove expensive. The early ones suffered a spate of coil pack issues and whilst most of these will have been fixed by now, a few will have slipped through the net and these are more likely to be the low mileage cars which haven't been used much. The TA will generally cost more to maintain than the 1.2; several of the service & routine wear and tear items are 2-3 times the price.
The two engines are very different and you need to drive both to find out which one is right for you; but be aware the TA will cost you more in just about every department bar road tax. If money isn't an issue, then the TA is certainly more of a fun drive, but if money isn't an issue, you probably wouldn't be considering buying a 7yr old car.
Whatever you decide, I'd recommend getting it professionally inspected; buying an older low mileage 500 with undisclosed issues is an easy way to lose a lot of money very quickly.
Also, be aware that many of the comments here will be from folks who buy new and trade away before the warranty expires; that is a very different scenario than buying a 7yr old car and being responsible for any unexpected bills. The best value car for someone in the first situation may not be the same as best value for someone in the second. Fashionability means used 500's generally command a price premium over similar cars of similar age and condition; IMO there are better value secondhand cars in the market right now. My own view is that the 500 is a car which is better bought new and traded in after 2-3yrs. If you can't afford the monthly pcp payments on a new one, you likely won't be able to afford the depreciation and maintenance on a used one, either.