I have a thread on the SunSun filters a little ways back. It explains a lot and goes through the unboxing of my 304B.
I still recommend you get two 404B filters. The 704B looks to be an overpriced 405B because they have all of the exact same specs.
Realistically, the 404B will probably flow around the 300-350gph range, once filled with media. Since you have a 120gal tank and it's good to shoot for a 6x turnover rate, two of the 404B's will be perfect. If you want, you can easily add a third one later. They are the largest filter that SunSun makes.