I will make a couple of assumptions here.
1. The Grizzly is new, with warranty, etc.
2. The Southbend is in pretty rough shape.
The question really becomes, are you prepared to do a complete rebuild of this machine?
To have the ways ground, you need to disassemble this lathe - pretty much down to the base casting.
Gibbs need to be manufactured and fitted.
Main Motor needs replacing.
I would suggest that the screws and split nuts need to be replaced on the top slide and cross slide (Been there and done that!)
God knows what else you will find at disassembly.
IMHO, it will be "cheaper" to rebuild than buy new. Until you factor in the time you will spend doing the work. So, unless you have more time than money......... I would buy new.