It depends on what area you're trying to absorb. If you're looking to do under 60 Hz I would probably recommend the soffit traps. But 70 Hz and above the four TriTraps would be much more effective.
The amount of reduction you get from the front or back depends on the room - sometimes one in each corner is best. I do recommend trying the different combinations (they are freestanding so aren't difficult to re-position. We usually recommend trying them on the front wall first as it helps cut down on some of the flutter echo and other high frequency problems in the front of the room a bit.
