Well, I honestly haven't compared the two, so I'd be a liar if I told you that one is better than the other. Didn't really fancy taking both of mine to Jesse's or vice versa. Anyways, I can comment on the list of features though. Maybe with that and your own ears testing both models you will come to some sort of conclusion.
From Mackie's site, with some paraphrasing:
1. Zero Edge Baffle - The HR824mk2’s imaging is stunningly accurate, with dramatically enhanced time domain characteristics and wide, even dispersion, delivering a much broader sweet spot than any other studio monitor on the market. This reduces what they refer to as edge diffraction, which can degrade the image. They also used a cast metal surface to reduces the surface vibrations during playback which can also distort the image. Metal is more inert than wood or plastic, so this makes a lot of sense.
Note, that the original HR824's have this. However, on the mk2's, the baffle was implemented with a rounder aluminum construction to further reduce edge diffraction. Also, the volume of the cabinet was increased slightly to improve low frequency response.
2. Passive Radiator - The HR824mk2 delivers outstanding bass response, thanks to a passive radiator design. Also, they mention that for conventional monitors, the ports or ducts must have sufficient surface area to prevent the velocity of the air inside them to exceed 5% of the speed of sound, which would then turn into audible noise. This causes a space issue in conventional designs. So the piston design of the passive radiator allows the mk2's to move more air than a port and therefore deliver a virtually distortion free bass response.
Note, that the original HR824's have this.
3. Amplifiers - Designed to provide the maximum acoustic output from the speakers, yet while minimizing the danger of overdriving them. The amplifiers' gain and frequency responses are hand trimmed to compensate for typical manufacturing tolerances. The connecting wires between the amplifier outputs and the drivers are kept to an absolute minimum. The acoustic sum of the outputs from the two drivers is optimized electronically, as well as physically, so that the amplitude response is unity and the phase differential is minimal.
Same in both models again. However, the amplifiers on the mk2's have been adjusted with all new 'voicing' for the larger cabinets.
4. Woofer and Tweeter - Here Mackie chose the best components on the market. Not too sure whether one set is better than the other.
Everything else is not too important, like the black gloss finish, which does look very dope, but doesn't really affect the sound.
* So the main differences are the baffle design, the cabinet size, and the voicing adjustment.
Mackie even states that if you have used the original HR824's you won't have to 'relearn' the mk2's, and you can get to work right away.
Will the new models help you produce better music? Doubt it, as the operator or producer is a key factor in this equation. I used to think that the newer the gear the better, but realized that is a load of crap. Use what you know and what gives you the best results. Of course, a shiny new synth will change or update your sound, but in the case of the HR824’s vs. the HR824mk2’s, I don’t think it will make a significant difference.
Hope this helps.