Adding bass to my PA system. Which option would be best? Both options cost the same. Just a wondering about sound dispersion.?

Has nothing to do with wattage. has to do with sensitivity and the SPL of the Bass unit. Bass is relatively non directional in the main. People decide where its coming from by attributing it to where the higher frequencies are coming from..just how brains work. Placement: Dispersion patterns, dead spots, other room considerations will decide what placement is best, by trial and error. Stacking bay well be the method of choice, or improbable, depending on the unit form factors. Since you did not note what brand and model the subs were, no one can guess how loud they play. I saw a huge custom built snail horn , which was probably incredibly efficient, literally shake all the theater seats, with the theater goers in them, during a premier performance of "Earthquake", a now old and forgotten movie.

One 350 watt will work better. Knowledgeable DJ's never place each sub under each main speaker. It is always best to group your subs in one place in the room - either stack them or put them side by side. You will get more bass that way and more even bass throughout the room. Here is a great article on the subject: http://www.prosoundweb.com/article/in_search_of_the_power_alley/

The whole point of only needing a single sub-woofer is that we don't need to place them spatially. You feel these as much as you hear them! Because of the large amount of air they need to move for the effect to really work a single 350W Sub woofer should work far better than two 75W ones! That only really changes if you were looking at an area in excess of around 1000 square feet - in which case a couple of 75W speakers would be lost anyhow!