Many, many differences between that bike and a Trek 520, they really are not comparable. Kind of like comparing a Chevy Impala to a Chevy Pickup truck.

Nothing wrong with vintage MTBs, they can make a tremendous value. I have several myself, and on my last vacation, when it came time to take just one bike, I took my $40 Univega Alpina Pro. Go figure.

Vintage MTBs get no respect in the marketplace (OK, there are a couple of rare exceptions), even though they would make suitable rides for 75% of the buyers out there.

As far as the bearings, they should just be loose balls. Sealed does not equal cartridge. Sealed tends to refer to the dust cap/shielding.