I think, having driven 500s with both engines (Parents owned a TwinAir 85bhp for 3 years & have had 3 1.2's as courtesy cars), I'd compromise on something like colour or options before looking to compromise on engine.
The 1.2 was a good engine, and some still are, but of the three 1.2s I've driven, only one (a 14 plate Lounge) was driving 'right'. That one drove nicely, but i'd still have taken the TwinAir instead, given a choice. The 14 plate Pop was a complete dog which blatantly had the well known flat spot problem - I had to thrash it everywhere, even on the slightest inclines, and it'd suddenly surge up to 5000rpm when finding the bite point, meaning I got no more than 44mpg indicated over 150 miles (about 8mpg worse than the TwinAir over the same route). The third, a 15 plate Lounge which I had a month ago was somewhere between the two, drove ok on the open road & got mid-late 40s mpg (so halfway between the dodgy 1.2 and the Parents TwinAir) but it'd still suddenly rev out of control when setting the clutch/accelerator at a junction. I nearly crashed it entering a roundabout as it wheelspan, which I've never done on a dry road before!
And therein lies the problem - the dodgy 14 plate was expected, as the 14 plates were when a lot of the iffy ones cropped-up, but then the second 14 plate was perfect and even more surprising, the 15 plate one was different again, but not as good as one of the 14 plates. The 1.2's since 2014 just seem all over the place with how they drive, and even if they do drive well, I think you'll still struggle to equal the TwinAir MPG unless you're content with going quite a bit steadier than you would do in a TwinAir - no mean feat when you can't get up a slight incline without dropping down a gear and flooring the accelerator.
The TwinAir gets a lot of flack in reviews but for me, if you've got the hang of its unusual power delivery & character, you're better of with it than the 1.2 if you don't mind the higher purchase price