On the knock-off z-tac side, the comtacs and the sordins are essentially the same, from what I've read. Common sense tells me the same thing.
Z-tac probably just takes its tech from the comtacs and puts them into a sordins case (or vise versa), so it's really just whichever one you like the look of.
In the real steel world, Sordins > Comtac II from what I've read, and Comtac IIIs are on par, if not superior to, Sordins.
As lurkingknight mentioned, if you look on google, you can find headsets for relatively cheap. But you'll need to swap out the mic with a low impedence mic - and depending on your PTT, may need to switch the connector on your headset. They come with a U94 or a 6-pin connector...most 6-pin PTTs are expensive and offer more than you'll need, so you'll want to either purchase a U94 connector, or get a U94 installed.
So drop $150-$250 on a used headset, $40ish on a mic, maybe $60 on a new connector, $40 for gel ear cups (if they don't have them when you purcahse them), and $150 on a PTT (or get one used for cheaper).
You're looking at maybe 4x the cost of a z-tac setup...but I literally have 5 z-tac headsets (I purchased them all used though), and all but two of them are broken. And of those two, their transmission ability is spotty at the PTT level.
I can't stress it enough - comms are a game changer when you're working at the team level. But if your transmissions are unclear (think of a cellphone with bad reception), or you can't receive transmissions clearly, then you're going to get frustrated, and it defeats the purpose.
If your gear doesn't even transmit or receive when you need it to, then you're going to be paranoid and frustrated - and so will your team. If the enemy shows up at your guard post, and you need to radio that in, you need your headset to work, or you're worthless to your team (a little bit of hyperbole there). If you're trying to raise your squad leader over the radio to get the go-ahead to initiate a coordinated assault, your team will be sitting there waiting for you.
But that's just my experience and thinking. You're already running your own comms gear, so I won't get into it further.