Two preamps are available....

Guys, that was just a general statement.  Obviously , boutique manufactures such as McIntosh will put a lot more effort into re-certifying and working to present "refurbs".  I did not now which particular preamp the OP was looking at.  There are some companies that will just be "does it make sound?  Then certify it!".

If its as reputable company the refurbished would be thoroughly checked over before put back in the box.  With usual quality control it will be checked,  but not always as much as a refurbished unit.  At least, that was how it was when I worked for a turntable company years ago.  The quality check can be more intensified with a refurbished unit.

Dealer, of course.

Used units used sparingly could be amazing. A used unit by someone living 10 ft from the ocean would be a problem as salt water corrodes electronics especially open designs like tube gear.
Even very good capacitors have a set period of time like 15 years after which a new set would be cool to have.
Refurbished units are also used units. No the caps are still the same unless they have been replaced.
Several good units are available from dealers that are refurbished to save you some money.
Look us up if you have more questions. www.sunnyaudiovideo.com

Are both preamps the same ?   Model, vintage ?

If price is the same, then the warranty is free.  I would verify the guarantor of the warranty....dealer (good luck), factory (great).

If the price drops on one of the units, then weigh how much a warranty is worth to you.