So you're going to use a slow flow sump, which is a great idea. I am still trying to determine which return pump to get...I like both the Iwaki series and the Eheim 1262. I am also wanting to get some more info/feedback on the Tunze stuff. Since I am going with a 4' tank, I was wondering if 2x6100 would be too much for it or if 2x6000 would be enough? Your thoughts would be nice...
Travis, I am going with a slow flow sump because of two reasons :1) I will be using a very large skimmer (IE, less flow means more time to skim) and 2) I will also be growing lots of macro algae in the sump, which prefers low flow.
As for the tunzes, I would recommend going with two tunze 6100s for your 120 gallon system. I ran two tunze 6000s on my 90 gallon, then ran two 6100s, which was overkill. However, your tank could handle it. I think Scott (Verper) is running two tunze 6000s on his 120g and he has lots of flow as well. Keep in mind that you can adjust the speeds/flow on the tunze 6000s and tunze 6100s, so either one will give you great results!
Both return pumps you selected are great! If you go with the Iwaki, make sure you purchase the Japanese pump (not American version). The Japanese version runs quiet and is built better, hence it costs more. You could go with the same pump I am using or the MD 40 RLT model, which is one size smaller.