1. Shopsmith spent years and years telling us that a two bearing quill had no advantage over a single bearing quill in everyday use. After a time all those imported clones were using two bearings as a big selling point so Shopsmith gave in and started selling two bearing quills. Not because they felt they were better but because the competition was claiming it was better and in our Tim Taylor world two is always better than one even when it isn't.
2. The rule here has always been to compare apples to sliced cheese. Nobody ever compares a new single bearing quill to a new two bearing quill. The normal constant here has always been to replace a well worn single bearing quill with a new two bearing quill and then declare that the two bearing is better because it has less play. Of course it has less play, the two bearing quill has a half hours run time on it and the old single bearing quill has a half zillion hours on it...
My mad scientist years taught me that such comparisons are pointless. If you want to prove it's better then you must compare a quill with a "new" single bearing against a "new" two bearing quill.
I have both two bearing quills and single bearing quills and I don't even have a clue which ones are which except my 510. I have it written down someplace and I guess I should look them up but as long as they both work properly I don't generally care.
Yes, A new anything is almost always better than a well worn anything.
It's fine to want to upgrade any machine but most of us shouldn't use "fuzzy logic" to make the decision. .