The thing about the 7x12" lathes and the small lathes in general is that you have to change the gears to perform different threading operations and this can be a pain if you do a lot of threading. Also they are not that rigid but will machine brass and aluminum just fine or harder materials in real small increments. If you get a 7x12", get the cheapest one you can find.
Get the 9" x 19" and use 3-4 people to move it. Bolt it to some cross 2x4's or something for movement. The 9x19 is a much more capable lathe. Threading operations can be performed quickly and the machine is more rigid. Speed changes require moving the belt, but that is a quick operation compared to changing the gears on a 7x12.
If you get a 9x19, you would want to get a 4 bolt compound hold down, as opposed to the 2 bolt that comes with it, as it is not rigid enough.
That is this product:
LittleMachineShop.com - Four Bolt Compound Hold Down for 9x20 Lathe
You also want to get a quick change tool post, regardless of the lathe you get.
LittleMachineShop.com - QC Tool Posts: Tool Posts