My question is this: can i use the USB option without having to use a laptop with it? At some point i may well get in to using rekordbox etc. or any software that requires a laptop, but for the beginning i literally just want to plug the USB in and start mixing.
The older Denon's link isn't very good. You could only see one drive at a time like on the Gemini MDJ1000 and sometimes had to restart the units when you changed the drives or the link state. The Pioneer link is vastly superior. You can see every drive on every other unit and it always works. You don't have to analyze your tracks to make use of a USB-capable Pioneer unit. You'll still get the same crude overall waveform you would on a Denon. There are also all the Hanpin standalone units out there that will give excellent usability, link, and at a more reasonable price than either. The only major thing you give up on those if you're not pre-analyzing files is the ability to offload tracks from the laptop, but if you haven't pre-analyzed, that's not going to work well on Denon or Pioneer offload function through Engine or Rekordbox, anyway. And the Hanpins have better pitch resolution. Denon's, in particular, was very poor res. Perhaps they could have improved it, maybe the link, too, but these are basically no longer supported by the new InMusic Denon DJ about to release the SC5000. I don't think the SC2900 or 3900 will ever see improvements again.
Also you write about pitch resolution (in several topics nonetheless) but fail to mention that is perfectly fine for manual beatmatching (certainly wouldn't describe it as "very poor" - that sound like comparing dry tehnical data without taking into account the actual use). Several of the problems you mention with link were resolved with firmware updates a couple of years ago (link).
And you are right about improvements on the SC units, they were discontinued a year ago (last firmware update was november 2014). But, Engine 1.5 is still being updated and that will result in better playback on the units themselves.
The Denon's link was a similar method to Gemini's. Have you used Pioneer's? Four USB flash drives can link simultaneously and all units can see them at will. No units need to be restarted. Totally carefree. If Denon finally improved their link at least not to require unit restarts, which was admittedly ludicrous, then that's an improvement towards the type of carefree functionality of the Hanpins that do it with USB cables, but doesn't come close to Pioneer.
For Link, Pioneer really does have it down. Ive had to mix and match 4 seperate model units in the past and they still worked. Connecting your laptop via link with rekordbox is a breeze.
I cant comment on denon's current setup, because we all dont have them to play with, but we shall see. Link on the older denon units was garbage.
Don't understand why all the hate about SC line of Denon units. Only thing I did notice was the higher rate of RMA's on the forums and such - Denon's quality control really slipped there. Even I had to get my mixer to the service because one microswitch was faulty from the box.