IMO, the sound between the 424mkIII and 424mkII is relatively the same.
You obviously get the pride of ownership, as first & sole owner of a 424mk3, when purchased as brand new gear, retail. Dealer exchange is usually 30 days and factory warranty is 1 year, which are important factors to consider, in case your Porta- has a problem, or you somehow change your mind about it. The used gear market has none of that.
You get a bump up in features with the [new] 424mkIII, definitely, but IMO the [used] 424mkII is still a nice unit to have, & despite the mk3, the mk2 holds it's own very well, feature-for-feature, and more importantly, in sound quality.
I think the make-or-break point between the 424mk3 and mk2, is that if you envision a more complex setup of INPUTS that may require the 6-full mixer strips, then the mk3 is for you. If you don't see going beyond the need for 4-full mixer strips, the the 424mk2 will suffice.
Another make/break, is that if you want your Portastudio to have a separate power module, [424mk3], or if you prefer the fully-self contained Portastudio with a standard AC line cord, [424mk2].
Maybe seeming like a nit-picky detail, but IMO, the 424mk3's external power module, wire and connector may be an added source of potential failure, way down the road, in the future. [... just from normal use & handling].
Self-contained power supply is what I prefer, [424mk2], but that being said, it did not keep me from getting a 424mk3 of my own. I just think it's a less desirable design feature to have external power module.
Note: I also have the 424mk2, fwiw, and I like it very much, equally to the mk3.
In most other respects, except the stark design differences, IMO the 424mk2 and 424mk3 are roughly equivalent in the hifi audio they can record and reproduce. Features may be slightly different, but specs are identical.
The 424mk2 and 424mk3 are the same class & family of device, just a few years and one revision different, and therefore they share most of the same internal components.
. And to think that I also got, with the mint 244, the original english language manual, an international manual with several other languages, a schematic of the 244, the original 244 color brochure, TASCAM catalog with all of their now discontinued gear, warranty card and a couple of other small brochures ... All mint for $154 USD! Downright criminal, ain't it ?? 
