Aye?
The 246 is best.
FYI, that's a typo, and it's actually a 424mkII.
The differences are many. What makes the 246 superior, overall, is that it's a 4-BUSS design, where the 424mkII is a 2-BUSS mixer design. You may have to become more familiar with recording terms to really appreciate that, but trust me, 4-buss is better than 2-buss. In the simplest sense, it enables assignment of any of 6 input channels to any of 4 "assignment" busses, which in turn, directly routes to 4 individual tape tracks. The 246 does this with a true 4-buss design. It's fairly superior to the L/R-stereo buss scheme of the 424mkII.
The 246 will enable you to assign 6 mixer inputs to 4 discrete program tracks, & onto 4-tracks of tape, live-in-studio, simultaneously. 4 discrete program tracks, live to tape.
The 424mkII will record EITHER two pairs of identical stereo tracks in BUSS mode [1-L, 2-R, 3-L, 4-R], OR 4 singular mixer inputs to 4-tracks of tape in DIRECT mode, with no front-end mixing, per se. You may NOT mix Buss and Direct mode recording, simultaneously, or if you do, your results will not be what you were looking for. The Direct mode on the 424mkII does not isolate the Direct channel from the L/R Buss, IMO a design shortcoming. IF the 424kII's Direct mode somehow was isolated from the Buss, then the 424mkII would equal the live-to-tape mixing capability of the 246,... but it doesn't.
Other differences are many:
246: VU meters
424mkII: Flouro-LED-type diplay
246: 2-band sweepable EQ on the mixer
424mkII: 3-band, w/2-fixed EQs and 1-sweepable EQ
246: 6 Full channel strips
424mkII: 4 Full channel strips
246: All [6] 1/4" input jacks, requires adaptor to accomodate XLR mic cables
424mkII: [4] XLR inputs built in, plus [8] 1/4" input jacks also
246: [6] mic preamps
424mkII: [4] mic preamps
246: [6] 1/4" Mic inputs, plus [4] Buss Inputs, total of 10 inputs
424mkII: [8] Inputs, incl [4] XLR-1/4" Mic inputs & [4]1/4" Line inputs, plus stereo SUB-In, total of 10 inputs.
246: [4] Tape Outs
424mII: [4] Tape Outs
246: DBX Defeat on All or Trk4
424mkII: DBX Defeat on Trk4 only
246: Brown, with a pitched meter section, & rear AND front mounted connectors
424mkII: Gray, flat, w/top mounted connectors
246: Inserts, which is an Out/In effex jack, all-in-one
424mkII: Efx Snd & Aux Snd, with extra inputs 5/6 & 7/8 for returns.
246: Loop function only
424mkII: Loop function with Auto-punch
246: [2] Headphone jacks
424mkII: [1] Headphone jack
246: No Direct recording mode
424mkII: Direct mode recording
There are probably more similarities & differences I've not covered. The main differences between the 246 and 424mkII lie in the mixer, as the 4-track tape section is largely the same. 6x4 mixer capability is superior to 4x2, and therefore, the 246 is usually judged to be technically superior.
The Direct/No-Direct issue is almost irrelevant, because you can still do more with a true 6x4, 4-buss scheme, than you can do with a 2-buss+Direct mode scheme. Think of it as this,... the 424mkII has Direct mode recording, almost in an attempt to bridge the gap between 2-buss and 4-buss operation,... but it's not quite technically "there". On the other hand, the 246 has true 4-buss mode operation.
The 246 and 424mkII are both nice Portastudios. I think the 246 is technically superior, with it's only glaring fault is not having XLR inputs, which the 424mkII does have. However, that's not a huge issue, as the XLR-1/4" transformer/adaptors are easy to get, and do a good job.
I'd also consider the 246's 2-Band Sweepable EQ to be technically a little better than the 424mkII's 3-Band, 2-Fixed and 1-Sweepable, EQ. I've used both, and the 2-band sweepable edges out the 3-band/2-fixed/1-sweepable EQ, just slightly. It's the 424mkII's 4-channels of FIXED Hi/Lo EQ that can make your mixes muddy, if over used. On the other hand, the 246's 6-channels of 2-band sweepable EQ gives you a great range of flexibility in "dialing" in those sounds.
The only other issue is age. The 246 is a much older machine than the 424mkII, and by simple logic, the 424mkII would probably require less maintenance in the short term sense. However, condition of used equipment varies greatly, from day to day, and I've seen many examples of mint or near-mint 246's auctioning on Ebay, that I'm sure have lived their entire lives in their boxes, in the closet. Or, the rarely used 246, that was pamered and covered it's whole life. On the other hand, I've seen many 424mkII's that looked like they'd been through the war. A quick mention of maintenance, is that when these machines fail, outside of abuse, it can usually be fixed by replacing some simple & inexpensive rubber belts. So, if you're a DIY'er, you may pick up outright steals on 246's, if you get one that needs a belt, but otherwise good condition. However, that being said, I've gotten my 246's off Ebay as mint & near_mint units, and I think that's better, & worth paying for. They were mint & ready to play, right out of the box.
Whether to get he 246 or 424mkII?? I have both. They are both good machines, that are easy to use, with respectable sound quality. They are both roughly the same budget range. The 246 has slightly more high tech built into it, than the 424mkII, which are technical things that make a fairly big difference in the 246's overal utility in actual studio use.
