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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. In other words, the "Guitar" input on the 414mkII is little more than a "gimmick". The only thing the 414mkII "Guitar" input gives you is an extra input jack and a "pad", (level control/attenuator). 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 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 piano key type buttons & the 424mkII has microswitch buttons. 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 2 band fixed EQ & the 424mkII has 3 band EQ with 2 bands fixed and the Mid band sweepable. The 414mkII has only High Speed & the 424mkII has Normal and High Speed.
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The 414mkII has LED bar meters & the 424mkII has a flourescent display with lots of detail. The 414mkII has 2 XLR mic inputs & the 424mkII has 4. The 424mkII/mkIII are a bit higher end in some features than the 414mkII, as you know.
