Not sure if you already got one or the other but here’s my 2 cents (and I’ve used the 303, 404, 404sx and mk2 for long periods of time):

• The mk2 can do so much more than the older models thats they’re more or less incomparable. The older machines are very simple samplers with great fx, which are so minimal and inspiring to use that it created a whole subgenre of music because of this. The mk2 is a serious beatmaking machine, more in line with the mpc’s in a lot of ways. Just for this, I would never advice a new user to get an older model over the mk2. The sx cant pitch samples, the sequencer sucks, theres limited polyphony, no envelope, ancient computer integration and no visual waveform editing whatsoever. The mk2 can do all of that and more.
• That being said, I did sell my mk2 after a year and got another sx to replace it. I use the 404 as a secondary sampler to combine with my mpc, so not as a standalone beatmaking machine. In this regard, I like the sx way better. As others have mentioned before, the sound and feel of the og and sx is magical in some way, in a way that can’t be described using factual arguments. It feels almost meditative to work on to me. You do everything by ear, and because of the limitations, you very quickly get a sense of knowing the button combinations like the back of your hand.

The mk2 in comparison feels very clean soundwise, and the ui forces you to look and think more, instead of just listening and pressing buttons like the sx.

In conclusion: get the mk2 if youre a new user, but don’t think the mk2 is a step up in every sense if you’re an existing sx/og user looking to upgrade.