The advantages of having at least two lathes is so great I'd consider rethinking the notion that you can only have one. I mean that even if two large lathes could not fit in the area available I'd sell the one that is the least accurate and then buy a smaller second lathe.
If one has a much larger hole through the spindle that would be a big factor for me in my own decision.
I'll second this. I have 3 lathes shoehorned into my shop, and no plans to ditch any of them. One is a Hardinge HCT, which is really my primary for most things, but it has limitations. The second is a large machine, a 16 x 60 Rahn-Larmon, which is an awesomely powerful beast, but big lathes don't have much feel and limited RPM ranges, so not great for tiny stuff. The third is a little Atlas 618, mostly decorative and to teach my young boys on, but I do use it periodically.
The Hardinge almost always dawns an 8" 4 jaw, and I use it for precision work, like barrel threading & chambering. But with no tail stock and a very small work area, there are times I need the capacity and other capabilities of the big critter. And even with both of those, sometimes it's more convenient to throw a small part in the 3 jaw on the little Atlas machine.
If I were you, I'd probably keep the better of those two machines. Figure out which has the best balance between capability and precision, sell the other, and buy a smaller lathe for small work and secondary ops. Something like a 10" x 36" South Bend or similar medium duty critter that's just easier to use, and which has higher spindle speeds for small parts and nice finishes.
Oh, and get a rotary converter. Statics not only cause severe power loss, but can actually hurt motors over time. You can get a brand new, US made 5 HP RPC for under $400.
