I don't imagine there will be much difference in price between the two, you will only be using 5 sheets of fabric, the F62 is slightly heavier duty, ie 6mm diameter rods arranged in 200mm squares instead of 5mm rods. My next door neighbor used F42 fabric on his driveway 14 yrs ago and it's still intact!! They used F62 in the slab of my house, so either way I would be surprised if you have a problem with your patio
The concretor knows himself that F52 mesh will only come back and bite him and risk his license.
The difference in sheet price is approx $3-$5 always go the correct size or one heavier .
As for the Tommy next doors drive way and not cracking there could be many factors contributing to this.
1) slab is extra thicker then required
2) The mpa of concrete is higher/stronger
3) No heavy loads have actually driven on the drive way
4) Saw cuts have been made in the correct areas
5) The base is compacted/prepared correctly
Never risk using smaller mesh then required as the repair cost of cracking concrete are huge.
If you have smaller mesh and want to use it simply double it up or strengthen it with extra reo bar.