I started with the PC 4212, went to the Rockler (bigger mistake) and finally settled on the Leigh and never looked back. I have no experience with the PC 77240 so I can't judge that one. It looks like their adjustable fingers are a copy of the Leigh system. I also think the Leigh RJT400 looks interesting if you have a router table. As with any of them their is a learning process so if you don't use it often you will have to refer to the instructions between setups.
A PC4212 is a very nice machine. Not anywhere near what I would call a 'mistake'.
Anyone that is comfortable with the 4212 will have no issues with the Omnijig.
The Omnijig is going to have better dust collection, better clamps, better fine tuners ('finer' tuners I should say), and the ability to adjust your machine for radius fronts on your boxes to an extent without having to resort to using shims and 'guessing' on your angles...
Unlike the 4212 - The Omnijig does not 'force' you to work in inch and 1/4 increments for things to work out evenly on the ends. You can tweak the stops or the fingers fairly quickly to allow for ANY size box up to 24" inches. Does not matter if the sides are 5", or 6 1/4", or 11 7/8", etc... Thickness not an issue either...
If I were to do this stack on my 4212 - I would have to wear a fairly long shirt / jacket lest my pockets WOULD end up full of dust / chips and the machine is going to limit me to particular increments on my sides for the ends to come out the way I want them with ease.
I get NO crap tossed in my pockets with the Omnijig.
I don't like the front stabilizer bar things on the Omnijig or the Leigh. They help teach bad habits in my opinion. The bad habit in particular being to raise UP the router after finishing the cuts. That habit leads to a jacked up template or bit sooner or later.
In the same way that you don't 'need' that stabilizer with the 4212 - You don't 'need' it with the Omnijig either. The dust collection will work just fine without it (providing you have a proper DC) and without all that crap in the way you can get in the habit of pulling the router straight out instead of up. (you don't jack up templates this way)
A stack of boxes this size = about a half of a trash can full of chips if using NO DC at all. I would estimate that I lose only about 10% of chips to the floor (or on me) when using my Omnijig with a 2HP DC fairly close to it and NO front stabilizer bar.
The template itself is rigid enough and large enough to the point that flex is not an issue and you have plenty of room for a full size baseplate on your router.
The dust collection on the Omnijig is good to the point that I don't have to bust out the 'fixin to shoot chips everywhere' gear for the girls.