It depends on your personal preference. That said, I'd probably go with MK7 given the choice. MK6 is often cheaper but more fiddly with all of the EPROMs and I/O boards, and getting replacement parts for them is becoming harder due to the machines being over 20 years old. MK7 on the other hand is basically a custom PC with less parts to deal with and a more robust and modern storage format (CF cards), but I have a soft spot for MK5 and MK6 as those are what I first started playing when I was old enough to enter a casino.
MK6 has a lot of classic non-progressive games but not that many standalone progressive jackpots aside from Big Red 2/Golden Samurai, Big Ben/Golden Gong, Lady of Fortune/Magician etc. (MK6's progressives were mostly link jackpots like Cash Express and Jackpot Carnival).
MK7/Viridian has a lot of progressives but can also run some of the MK6 library which were ported to MK7 virtually unchanged as "legacy" software or even remade in native HD (e.g. Lucky 88 and More Chilli), but not all MK6 games made the jump to the newer platform e.g. the Lil' Lucy and Money Honey titles are MK6 exclusives AFAIK. Of course, there are also a decent chunk of MK7 exclusives too, of both the non-jackpot and jackpot varieties. If it's a Viridian WS machine you've also got some MK8-era titles to work with like Lightning Cash.
As for sound, the earliest Viridian games (both legacy and native) have the exact same sound quality as MK6, however later titles have better sounding music, although in both cases it's basically just MIDI quality but with a different soundfont.