Suppose you have the opportunity to acquire a used Martin or a used Northfield planer (both 24"). Northfield has a helical head; Martin has a tersa. Both in good condition though the Martin is probably in marginally better shape. Price close enough to not be a factor. I'm making table tops and conference tables.
I'm leaning towards the Northfield because the absolute precision of the Martin is not essential, and I have a hunch that Northfield parts will be easier to find. But I'd be easily persuaded otherwise.