Yuppers.. Gonna Agree with Komodo here.. There is a price diff. close to $10.00
Personally I hate HOB filters, however on my 125 I do currently run two Emp.400's, along with an XP3.
Don't get me wrong, Komodo brings up a VERY valid point.. Everyone will ALWAYS have there own opinion. With the 350, sure you do have less parts to play with, an sure that can be a good thing.
However with the 400, like mentioned before you do have the spray bar, that you personally can control, setting the speed of how many rotations the actual bio-wheel will spin, rather then the 350 where your leaving it up to the water movement leaving the filter. I HAVE seen that you have to keep an eye on the 350's to continue to check if the wheel is moving, otherwise killing off the beneficial bacteria.
On that note, with the extra cartridge in the 400, as komodo mentioned, you have the option to add pretty much whatever you want. In his and my case, BioMax. You could just add another blue sponge/carbon.
Also, it is larger.. you will always have MORE water housed in the tray at any given time.
For me it was an easy decision. For close to $10.00 I know I am getting the better filter (personal). I never have to worry about the wheel, I have better Bio filtration, and more water capacity.
It's both you coin, and your tank.
Run what you wish.
I hope this helps.
Taylor