i looked at the official best acoustic guitar under $300 thread but those are acoustic guitars and i am interested in buying an acoustic-electric guitar. My style is classic rock and grunge and some metal. Also, will an acoustic-electric guitar work well with my line 6 spider III amp?

P.S. Please do not tell me to get a new amp. I understand how UG hates these things but i am not ready for a new amp. Im happy with the spider and i dont plan on getting rid of it anytime soon.
an acoustic electric that cheap isn't going to be great... unless you REALLY need to run it through an amp for some reason, i would say spend the full $300 on a regular acoustic.

i've had 2 acoustic electrics that i've run through a small amp, and the guitar itself will most likely drown out the amp (unless you have it cranked) and it definitely won't sound great unless you have a real acoustic amp.

but like i said, for $300 or so you might not find much. i recommend an oscar schmidt if you really need an acoustic electric. they sound decent enough for the price, and the electronics could be worse. but they'll be laminated wood with no-name electronics in them. check them out, if you like the one you try as much as i liked mine then you made out with a good deal. i talked them down to $200 on mine, so try that too.
Warmoth Telecaster Deluxe. Warmoth Strat. Seagull Artist Portrait Acoustic.

"Well good God damn and other such phrases, I haven't heard a beat like this in ages!"
-Dan Le Sac Vs The Scroobius Pip
About the best your going to do is an Alvarez RD20SC. If you get something like that get the saddle and bridgepins replaced with bone as the plastic ones on it are crap.
If you're willing to go a little higher, Seagull sells one for $429

I have the mini jumbo pure acoustic, but have played the ae version as well. But like the guy above me said, unless you're seriously gigging, you probably don't need electrics. Miking generally sounds better until you start to get into the $1500+ range, when manufacturers are putting in quality electronics and not just the cheapest they can get.