My opinion, please note that I do not bash other companies because this is a very subjective hobby. I own two UFW-10's and they are tight and of great build quality with a real wood veneer, that may not matter to you. I have heard the ULW-10 and although it is not a UFW it is quite good and has the same build quality.
Now as to your question about the watts, the amp on the Energy is a 200W amp, the 800W claim is not the actual continuous wattage, but the dynamic peak of the amp, please forgive me if you already knew this. Most manufacturer's, AFAIK, do not list the dynamic peak of the subs amps. I have never heard the Energy sub so therefore I can not comment on how low it can go or how good the bass sounds, Energy does claim it can go down to 21Hz which is lower than the ULW-10, 26Hz.
Where I am confused is the comment you made about the package that the installer has, what speakers are coming with the Energy sub for $500? If it is the TAKE system, then $500 for the sub and the satellite's seems like a pretty good price, MSRP on the sub alone is $500. The ULW-10 will cost you around $399, family pricing IIRC, so for the $500 you quote for the package from the installer you will be getting speakers and a sub? Like I said, part of your post was a little confusing to me so I am sorry if I misunderstood.
To conclude the ULW will not go as low, it has a higher rating for the amp at 350W(continuous) and is finished in a real wood veneer. The Energy can go lower, I assume has a vinyl finish and is 200W(continuous). The review(Home Theater) I read about the Energy claims some boominess was detected and it weighs about five pounds less than the ULW-10. The ULW-10 won a "Best of 2005" from Audioholics and their test claims a flat response down to 24Hz. If you were comparing the ULW-10 to the SVS PB10-ISD then I would say it would be a tougher call for me to make, I do not think that the Energy sub is in the same class as the ULW-10. If the installer is willing to offer a 30 day return then you could always grab both of them and decide for yourself. If you do not like the ULW-10 then you are out your shipping costs. Good luck and happy shopping.