The monster unit actually weighs about a pound more. The last time i fell for one of those "Automatic voltage regulation" jokes was when i got a battery backup for the pc, It was actually a unit that was for home theater too. Voltage regulation my ***, Their definition of voltage regulation was having the thing kick over to battery power which outputs around 95 volts. Great!
If your just buying it for the surge protection and filtering Personally I think I would choose the Monster. I had bought an HTS3500mk2 just less than a year ago for pretty cheap off buy.com ($109), When the volt-meter illumination gave out a couple weeks ago i called monster, sent it back, They sent me a brand new Hts3600mk2 for free. No hassle, Quick turnaround. The monster has a 5 year warranty and at least from my expeirence they honor it no problem. Its also got a higher Joule rating, and "t2" circuitry which from their descripton saves your MOV's (the things that do the surge protecting, by sacrificing themselves) from failing prematurely. I think the monster is also probably capable of more current delivery. I read 12A max for the APC unit on one site. Not sure if thats true or not.
Heres a pic of the 3600mk2 Insides just so you get an idea of what your buying...