The G3 is much simpler to maintain, because the gun itself is much simpler. It literally has one moving part internally, and it comes out by simply unscrewing the back cap. That's not to say that the Mini is complex, but there's certainly more stuff inside to worry about.
The sound signature on these two guns is backward. You'd normally expect the spool valve to be the quieter gun, but this is not the case. The G3 is louder, there's no question. In fact, the G3 is louder in comparison to quite a few markers. The Mini, on the other hand, is rather quiet. It's about the same volume as your standard Autococker (without the sound of the moving pneumatics, obviously). Anyone who says a Mini is loud either has a defective marker or is WAY too picky.
Out of the box, I prefer the feel of the Mini, and this is mainly due to the trigger. The Mini's stock trigger is pretty decent, while the G3's trigger sucks pretty hard. The Mini's magnetic return makes for a pleasant, easily walkable, fun to use trigger. The G3's trigger gets the job done, but it feels more like a tool than an instrument of passion. To some degree, this can be fixed with an aftermarket trigger (I highly recommend a TBO), but the tool-like feel never really goes away completely. I like the balance of the G3 better, and it's grip frame is more traditional and ergonomic. Both guns are very light, but the G3 honestly seems to weigh nothing. The Mini's not heavy by anyone's standards, but it seems to have more significance to the feel of it when compared to the G3.
Neither gun kicks very much at all, but the Mini probably has less of it. Personally, this is a non-issue. Frankly, I find the idea of "kick" on a paintgun laughable. If you want kick, go shoot a 12ga shotgun with a slug loaded in the chamber or any rifle loaded with .300 Win Mag or bigger.
But back to paintball. In the efficiency department, the Mini probably takes it out of the box. However, with a little time and tuning, the G3 can show some pretty competitive numbers. Neither gun puts up remarkable efficiency numbers, but they're not gas hogs either.
In the reliability department, there's a little hitch. When the Mini first came out, it was fairly hit or miss whether you'd get a good one or whether you'd get one that'd give you constant headaches. They fixed the issue quite some time ago, but those troublesome Mini's are still out there, and predictably, they're usually for sale. So if you're in the market for a used one, you've got to keep that in mind. One of my friends bought one of the temperamental Mini's. It was giving him nothing but trouble for a long time. Luckily, he was able to chase down the problem and eventually fix it. It now runs great, but it was a pain to get it there. Another guy on my team also has a Mini, but he's never even so much as chopped a ball, as far as I know. As for my G3, it's been infallible since the beginning. I once even played an entire round with only one grip frame screw (an issue that was my own fault), and it still shot perfectly, even with me having to hold the two pieces together.
Both markers have pretty decent regs. I've not heard any issues with consistency from either. There used to be a problem with the G3 reg where is was possible to crush the reg seat if you were an idiot, but they've since fixed the issue and made the upgraded seats available to owners of the crushable ones for the cost of shipping. In the area of ASA's, the G3 wins hands down. The flip lever on/off (DP calls it the RAPS) is one of the best ASA's on the market today. Besides being really cool, it's incredibly easy to use and perfectly effective. Compare this to the Mini's ASA which contains the reg and has no on/off feature, and the G3 wins hands down. However, the Mini has no macroline to deal with, while the G3 relies on the standard configuration of a hose running from the ASA to the reg.
The Mini's got more aftermarket support, but there are two reasons for that. The first is that it's been around longer, and the second is that the G3 doesn't really need anything. Outside of a trigger, you don't really need anything to make the G3 a great performer. Mine has a TechT bolt, an TBO trigger, and an APE Upgrade board, and I consider it to be complete.
I can't think of any other comparison points off the top of my head, but if you have any questions, feel free to ask and I'll do my best to answer.
This post has been edited by Corrupted355: 03 March 2009 - 10:03 PM