dennnis...
The 424mkII/mkIII are a bit higher end in some features than the 414mkII, as you know.
The 414mkII has 2 XLR mic inputs & the 424mkII has 4.
The 414mkII has LED bar meters & the 424mkII has a flourescent display with lots of detail.
The 414mkII has only High Speed & the 424mkII has Normal and High Speed.
The 414mkII has 2 band fixed EQ & the 424mkII has 3 band EQ with 2 bands fixed and the Mid band sweepable.
The 414mkII has an AC/DC power module & the 424mkII has an attached AC power cord, (w/424mkIII having a detachable inline power module).
The 414mkII has piano key type buttons & the 424mkII has microswitch buttons.
The 414mkII has simple operational functions with Zero-Stop and no frills & the 424mkII has sophisticated RTZ, Loop, Repeat, Memory Locate and Autopunch operations built in.
The 414mkII has a dedicated "Guitar" input, and the 424mkII/III has no comparable feature, but (hint) each Mic/Line input on either the 414mkII or 424mkII can handle a "guitar" input. The only thing the 414mkII "Guitar" input gives you is an extra input jack and a "pad", (level control/attenuator). Nothing else. In other words, the "Guitar" input on the 414mkII is little more than a "gimmick".
In sound quality alone, the 424mkII/III should enable you to make a slightly better sounding recording than the 414mkII, in part by virtue of having a slightly better channel strip EQ section. The 414mkII should make a decent and comparable recording to the 424mkII/III, but the 424mkII/III also have a totally different electronics section and audio mixer than the 414mkII. The 424mkII/III would be more high end and finely tuned mixer and audio section than the 414mkII, but the difference in real audio sound quality is very minimal, with subtle differences perhaps audible to the acute or trained ear.
Choosing between the 414mkII and 424mkII there are many factors, but it's not between a bad/good choice. It's more like a good/better choice of factors.
One factor that's not equal is that the 414mkII is still available as a brand new retail product in some venues for a limited time, but the 424mkII and 424mkIII are discontinued eBay-only type of items.
Q: By chance did you catch my SC homepage featuring several recordings off the 424mkIII and dozens from the 424mkII ??? Just wondering, but they're out there. I've been able to get a good sound quality out of both of these units, the results of which are featured on Soundclick. FYI.
The 414mkII is decent, and is very well geared toward "backpack" remotes and casual-to-serious use in productions. The 424mkII/III is really more of a production machine, which then were both top-of-the-line. You'll generally find the best features and sound quality on the most high-end units, by virtue of better designs and components used in the topline offerings of the time.