Well first of the 8x12 is a much beefier stronger and better Lathe then the 7x12.
I believe the reason the 7x12 is more popular is that it has been around for a much longer time then the more recent 8x12 as far as publication is concerned.
There are a lot of vendors of these Machines and the differences in Pricing makes it worth to shop around a little.
The 8x12 is generally quite a bit more expensive then the 7x12 roughly double.
So your choice would depend on what you are willing to spend.
The size of these Lathes is close to equal and there is nothing the 7x12 can do that the 8x12 could not.
Most other accessories are totaly interchangeable between these Lathes and others in this class.
The 8x12 is also much heavier wich is a good thing in Metalworking machines.
Shopping around for a vendor wich offers good Service after sale may also a consideration.