TA-11 is my favorite ACOG by far. Ermagerd I want one with ACSS so bad. Trijicon hear my prayer!
We've helped a lot of guys decide between the 4x ACOG and the 1-8x Platinum. I think you've correctly identified the distinctions here. Battery life on the 1-8x Platinum isn't great, you're looking at only about 250 hours at a medium setting. That's what the manufacturer can accomplish right now, at least we are being honest about it. It's not crazy bright either, but at least you get a smart reticle illuminated. The Vortex Razor and Leupold Firedot are brighter, but they aren't really illuminating their reticle, they are just giving you a center dot and nothing else in low light.
Durability on the Platinums is what you would expect if you know that manufacturer -- excellent. My coworker Will has one on his SCAR17 and it's a great combination there. Those SCARs are known to be scope killers but the Platinum is chugging along just fine.
The best thing I can say about the Platinum is, looking through it back to back with the 4x ACOG ACSS, the glass quality really is very comparable to my eyes. Meaning the glass in the 1-8x is delightful to me.
Trijicon lists the TA11 eye relief at 2.4 inches, so we have it beat there, the Platinum's eye relief is 3.9-4.0 inches. For high recoiling rifles that does matter, if you're wearing nice eye protection 2.4 inches isn't much before you smack expensive glass against expensive glass.
TA11 exit pupil is 10mm at 3.5x magnification. I freakin' love the eyebox on the TA11. Platinum's exit pupil is 11.7mm at 1x and 3mm at 8x. At 3.5x, exit pupil will be pretty close to the ACOG's, with a very forgiving eyebox. And at 1x the eyebox for the Platinum is superior. At 8x, you'll want to be supported from a bench or in the prone. Shooting offhand at 8x will be frustrating.
And this leads me to my biggest selling point on why I will personally get the 1-8x instead of the 4x ACOG ACSS for my FAL. A 1-8x LPVO is so much more flexible than the ACOG. For up close shooting at CQB range it is noticeably faster to crank down the Platinum to 1x, have that reticle shrink to a "donut of doom" and keep both eyes open. At 10ft to 25 yards my brain likes that much much more than doing the Bindon aiming concept through a 4x or even 3.5x. If the ACOG was that great in CQB they wouldn't have sold a metric boatload of $1400+ ACOG/RMR combo scopes to the US military. On the other end, we had both the Platinum and the 4x ACOG ACSS at Media Range Day at SHOT Show this year, and we were able to get hits at 500 yards with both setups. But even at just 500 yards, it was WAY easier to get hits on a steel torso when you literally have double the magnification available on demand. With the ACOG you make do with your fixed power no matter what situation you are shooting in. The flexibility of the low power variable design really makes the 1-8x superior up close and far away, without really losing much ground in the middle either. If you really really like 3.5x magnification, you can set the Platinum there as your default and only touch the magnification ring in certain shooting situations. No problemo, but the added capability is there if you need it.
If you want something as small and light as possible and don't want to ever have to worry about battery life, and the flexibility doesn't matter that much to you because you aren't really planning on using an AR-308 in close quarters anyway, then maybe the ACOG suits you best. You also save some money because you don't have to buy a mount separately, so there's that. The Platinum has a hidden cost in that you need to add a good 34mm mount (I like American Defense for the money, BOBRO if you're makin' it rain and cost is no big deal to you). The ACOG is combat proven all around the world, it has pedigree we can't match.
But, for me the flexibility of the LPVO design wins. If it wasn't PA, for the same money I would look hard at a Razor HD Gen II or even Trijicon's own Accupower 1-8x before I would do the ACOG. One man's opinion and I hope you find it helpful.