These two machines appear to me to be light years apart. For starters, one is a 10"; the other a 14". Both seem to have similar bed lengths - 48" sounds a bit long for a 10" lathe, but from the picture, that looks accurate. The 10" only has a 1-1/16" spindle bore; the 14" has a 1-3/8". Neither is huge, but you can obviously do quite a bit more with the larger bore. The Rockwell is definitely a much heavier machine. The Clausing appears to have NO tooling - not even a chuck or a tool post. That can run into a lot of money before you're able to turn anything. The Rockwell has some tooling - including a 9" 3-jaw and a 9" faceplate and various other pieces, but from the pictures and the tooling included, it possibly has been rode a lot harder than the Clausing. The biggest difference is likely to be the motors. Neither ad gives the HP (some of our seasoned veterans may know), but the big factor is the Rockwell is 3-phase. If you don't have access to 3-phase, you're talking a pretty good bit of money just to be able to turn it on. The Clausing doesn't say what size/phase it is, but my guess (and it's purely that) is that it's single phase. And without examining them, it's challenging to try to ascertain how heavily used or worn out either may be.
I suspect you'll find that either of these machines will likely require a lot of work and/or money. If you're looking for something to plug in and use, be careful. If you would like to work on the machine (which, in my opinion, is a great way to learn how to use it), these 'could' be okay - but it could cost a lot of money.
Price: Probably a whole lot closer than the difference in asking price. Chucks, tool posts, steady rests, tooling, etc., etc., etc. can get very expensive very quickly. You'll find plenty of threads on this site that accurately point out that the purchase price of the machine is just the beginning.
My advice: Be careful. That doesn't mean don't buy either machine. But if you choose one, EXPECT that you'll need to do some work and spend more money. I've bought three lathes and a mill that way. One of the lathes and the mill were huge disappointments AND money pits. The other two lathes were fine.
Regards,
Terry