As far as an "apples to apples" comparison, in my mind the Montana would be more comparable to the Designer, so (as I would have expected) you may be able to get farther upmarket for your money with the used Montana. We've owned three (3) Jaycos in the past, two (2) of which have been Designer 5th wheels. All have given us years of enjoyable service, and Jayco has a well-deserved reputation for standing behind its products.
Having said that, if the Montana has been well maintained, and if it has the features you want, at a $5K discount relative to the Eagle it could be the most bang for the buck as I mentioned above. Your money - your choice.