My advice is to get the cheepest one you can find. Unless you know the nitty tech specs on them... Most of them are modified sign wave output, which means that they don't make a sine wave, they make a positive square wave followed by a negitive square wave. If you can find one that shares a common ground with the minus side of the battery, that is the way to go. The ones I have had could not be grounded because the ground and neutral terminals were not connected right through to the negitive side of the outlet.. I don't know why they didn't do this, but I know this is the reason people have their inverters fry up. If you get your inverter and measure the gound output with respect to the chasis of the car and you get anything other than 0 volts, (I gor 40 volts AC) then if you try to ground your case, you will have problems. I used a non grounded mpegbox off an inverter for 2 years with no problems at all, I got that inverter for 20 bucks from Frys... A non modified sign wave output isn't that important unless your power supply has issues with filtering and such.
My 2 cents,