I have used both. If anything, I think Me-1s are the easier to use for the musician because the Me-1 gives more information about which signal you are editing (importing channel names from the mixer) while the Me-500 just provides a generic, “let’s see, what was that signal in channel one…Was it kick drum or lead vocal?”
I think the street difference in price is closer to $150.
You say you need 12 units. What really makes a difference for Me-500 is how many of these 12 units are going to how many different Types of Musicians. Let me see if I can clarify this with some examples. . . I have a singing drummer who needs good monitoring control over his individual drums and his own vocal other individual vocalists. What if this same drummer is in a band that has a bunch of horn players who want control over individual horns, and a bunch of singers who want control over say 5 lead and background vocals. In order to meet the needs of all these players (AT THE SAME TIME), I’m trying to fit 5 vocal mics, 4 horns, 8-9 drums … wait a minute, I’m already over 16 and none of musicians get reverb, nor are any of them hearing bass, keyboards, ambience mics, talk back, etc, and I haven’t even begun to have mixes for the actual bass player, keyboard player, etc. Yes, with Me-500 you can create groups on your mixer, like Drums, Horns, Vocals, Keys, Rhythm Section, but your mixer may not allow you to make all of these groups at the same time. The 40 inputs of the Me-1 begins to look much more appealing, as does the ability to create groups on the actual Me-1 unit.
I think Me-500 makes sense for bands who can really work easily within that 16 mono or stereo inputs limit. Let’s say a band that only uses 4 drum mics, bass, 1-2 guitars, 3-4 vocals, one reverb, 2 keyboard channels. But if you need 12 units, you are already saying you got 12 people, so it quickly begins to sound like Me-1 territory to me. But if six of these units are going to a choir or horn section, that is different than if all twelve units are divided equally among very different musicians. Tell us more about your situation.
Yes, you could get some Me-1s and some Me-500s and dedicate the Me-1s to those with more complex monitoring needs. But the Me-500s would then dictate which inputs are in slots 1 thru 16, a lowest common denominator. That might or might not be a problem, depending on the flexibility of your mixer’s routing and the number of inputs you actually have (e.g., local plus stage boxes).
Musicians also like the local ambiance mic that is only on the Me-1. I could make the me-500 work in larger contexts, but it made my head hurt, and the monitor engineer really has to master the mixer and pre-think through the routing. If you only need to do this once, that is a whole different situation than if you need to reprogram the whole thing often for different bands, musicians, gigs, etc.
For me, that $150 difference seemed trivial in hindsight, but I only bought two, drummer and keyboards. My other current players get wireless IEMs from Aux mixes and do phone editing of their mixes.