Well i'm running 165/60/R12's on my 12x5's and they fill the wheel nicely and look a little fat..certainly not 145 cheesecutters
If you go 12x6..you be using the same 165/60 tire and sure the rim will be wider..but it's only an inch..and IMO i think the tire won't look as good..
I have [very bald] falkens at the moment and as such i'm going to be forced into getting some A539's to chuck on until i decide what i'm going to do with rim choice.
12x6's aren't available here, probably possible under custom order from superlite..$$..
For the effort you would go to even to get them from the UK, i don't think it'd be worth it over the 5..
If I was you and going 12's..i think you can still get 5.5" and they are what i would choose.
Since i already have 12x5's..i'll probably keep them..unless a 12x6 tire is available..
It's really a question of what your after.
I've driven mini's with 10x5, 12x5, 13x6, 13x7 and you could argue the handling goes from left to righ [10x5 being best -> 13x7 being worst] to looks going the opposite way.. 13x7 looking the best...10's least..but that's oppinion only and 10's look good i think..
Only reason I run 12's is because i have them..I don't think 10's look good on clubbies...you can't fit 10's over my brakes and I don't like the feel of 13x7's...13x6's are OK though..
The bigger the wheel, the less mini like it feels and you loose the 'chuckability' ...bigger wheels make it feel more like a modern jap car..
my..10c