Power consumption is a trade-off with the SunSun filters. My 303B uses 35W for the filter and 9W for the UV for a total of 44W (unless the 9W is included in the 35W, but I assume it is not). The larger 304B uses 55W and 9W for the UV for a total of 64W. The UV can be turned off with a switch if you want to save power. The Fluval FX6 comes in lower at 43W. For the price difference, I think this is acceptable and would still recommend the SunSun over the Fluval. I also think that having the UV built in is a benefit. The water flow is probably too high to do a great amount of "sterilizing" but it will definitely help keep the water clear.
Btw, to put the wattage into perspective, the smallest regular light bulb typically found in the house is 40W. A lot of times your light bulbs are 100W+ and probably average around 75W. The SunSun HW-304B is still probably less than your average regular light bulb. If you are really counting watts then maybe the Fluval will work better but I think it will take many many years, maybe decades for the cost difference between the two to be made up in electrical savings. Also, when comparing the Fluval, make sure you compare the working flow that is about 500+ gph instead of the advertised pump flow of 900+ gph. The Fluval will probably flow a bit more but I don't think it is a deal breaker by any means.
JMO, but I don't see any wrong choice between the two.