I recently bought a Panamax 5100 for my system. Like you, I did not know which way to go. The Monster units look much sexier but cost a couple hundred more. The decision was made when I e-mailed each one with some questions. There was a huge difference in their replies. To make a long story short, I bought Panamax.
Oh, when I buy a line conditioner/power filter that says it can do XYZ. I like to check and see if it really can. First, I have a digital Sony HDTV (40" XBR800) along with a digital Hughes satellite decoder + DVD + CD changer + 70w x 4 stereo receiver. I live way out in the boonies and have a back-up diesel generator for the house. To test the system I switched the entire house over to the genterator; turned on several lights, got the fridge to kick in, ran some hot water to make the elect. water heater run and get the well pump cycling on/off. Nothing hooked up to the Panamax noticed a thing. The picture on the TV never flinched, no specks, no distortion, and there was no noticable change in the audio. In other words...perfect. I went down and dropped the rpm's on the generator to cause a low voltage low frequency situation. Still, everything connected to the Panamax worked perfectly up to the point where it's low voltage protection kicked in. I had the voltage & frequency so low that plain old light bulbs were noticably flickering but the goodies connected to the Panamax didn't notice a thing. Once I bumped the voltage & frequency back up the Panamax turned back on automatically and everything was fine.
PS: Don't mess around feeding your expensive gear crappy power unless you are a gambling man. Did I mention that I love playing craps.