And here is the difference.
A 10" Contessa with 165/70R10 versus a 13" Superheavy with 195/45R13....(there is only about 10-15mm difference in rolling diameter)
But beware, going 13" rims to fit bigger brakes is a sensible (IMHO) option, but it does come at a penalty.......
1. Increased unsprung weight (12 kg's per corner with tyre!)
2. Higher wheel frequency (unsuited for rubber cones, which like low wheel frequencies)
3. Increased leverage on the bearings (premature wear and replacement)
4. Potentially wider offset, means bigger flares (13 x 7)
5. Potentially wider offset means trimming the guards (13 x 7)
6. Potentially wider track my negate road legality
7. Wheels may be dearer, as will the tyres
But, for every penalty, there is a benefit................
1. Able to run bigger brakes (up to 9.5 inch if you went Wilwood etc)
2. Larger contact patch means better grip
3. Larger contact patch means better frictional braking (if brake swept area remains constant)
4. Larger choice of tyres
5. Larger choice of rims and offsets
6. Makes a Mini look more modern (purely subjective)
7. Rolling diameter slightly taller which means lower revs at hwy speeds
Your choice, but I chose 13's because of the look and stance it gives to the car (subjective I know!), and it gives me greater flexibility when it comes to tyre choice, brakes and the rims themselves. But, if you want that period Mini look, ditch the 12's and stay with the 10's. I wouldn't put 12's on at all, either 13's or stick with the 10's.
Any reduction in handling can be potentially offset by better suspension parts and design.
My $0.02
Cheers,
Tricky
I had to sell my 12's because of the brakes, but i have A048R's on the new 13's 
