What goes into Bs improve all the time such as pumps, inverters, converters, refrigerators, etc. The 8 year old B may have better. At least that was the case between my 2005 and 2011 class Bs.
The 12 year old B quite possibly has original tires if only 30,000 miles. They may even look good but are 4 years past reliability on the open road. The newer B probably has had new tires installed by now after 60,000 miles. That's something to consider.
Also, a B that sits wears out faster than a B that's used. That's similar to an abandoned house that falls apart rapidly and a lived in house that constantly gets tended to.
A low mileage B if it was used regularly all that time without other extenuating circumstances could be an indicator the B was not all that comfortable to use and didn't meet expectations by the owner.
The above are some given suppositions to consider all things equal. The bottom line is there are still a lot of variables and the decision can't come down to age and mileage. You have to inspect both, drive them and imagine how you would use them to make a determination.