After selling my previous phone and buying an Indulge off someone who only used it for 4 days...gave me his receipt and everything...ended up getting an Indulge for much closer to $300 like I wanted. That's a pretty decent price for the phone in my opinion.
But $399 + tax for the Indulge is a rip if you're coming from an Optimus. I don't know what your situation is as far as Internet connectivity goes though. Are you around wifi a lot during the day? Do you have Internet access on a desktop/laptop at work all the time? If you answered yes to either of those questions then there's no point for you to get an Indulge from an Optimus.
Oh and as far as hardware differences are concerned...you already know the only real noticeable one is the processor speed and I think the Indulge is running a better video card too. Don't know if you're much of a gamer, but I've already tried about 7 - 8 graphic/hardware intensive games and while I got some to work, most of them either didn't load or crashed the Indulge. The best looking ones I have installed that run smooth are Need for Speed Shift and Gangstar (GTA knockoff). The Indulge's zooming speed when you use multitouch during web browsing is much faster/smoother than it is on the Optimus...but that's not really a big deal. So it's not like you'd be getting THAT major of an upgrade in the hardware department with this phone either.
If I were in your position, I'd keep the Optimus and wait and see if they release an LTE phone w/AMOLED, flash camera, and Android 2.3. If you just can't take the slow Metro 1X speeds any longer and aren't around wifi that often...wait for the price of the Indulge to become more reasonable and then buy it, or get a barely used one like I did.