Always assumed cassette decks had a plethora of belts to make things work, but recently saw some direct drive models listed for sale.  My experience is that rubber belts perish, stretch, etc and need replacing.  So, would a direct drive model have advantages?

Sub,

Yes, broadly speaking from that point of view, single capstan DD decks at least don't have a drive belt and are technically, supposedly, more reliable. DD Dual capstan decks usually have the feed capstan belt driven off the DD take up capstan. ( Nakamichi Dragon excepted, and possibly one or two other esoteric decks with a dual DD drive). The main reason I feel for for going to DD motors was to reduce W&F further and have a 'techy feature' in the never ending game of spec'smanship. Sure belts perish and stretch, but are not usually a major mission to replace, and for the spec's.. beggar me to hear the difference .  Most of the later/ last produced decks seem to have reverted to belt drive on cost grounds.. at least the ones I've been exposed to.. and instead opting for Dolby S to upspec.

What almost all decks have is a rubber idler/spool drive, and when that goes brittle, they are often much more of a drama to replace than the main belt.

Never play Leapfrog with a Unicorn.

someone needs to do a nak zx7 (belt)  vs. zx9 (dd) comparison.

yes, but that is a little bit apples and pears... not just the DD transport that is a difference... and I don't have a ZX-9 on the fleet 

Never play Leapfrog with a Unicorn.

The ZX-7 has an all-bipolar transistor tape head amp; the ZX-9 has a jfet input tape head amp. So they might be sonically slightly different, apart from the transport differences. You would have to put that aside and listen for wow and flutter differences only.

AMR-iFi R&D