There's not much difference at all. It's like calculating the differences beteen extra strength aspirin and extra strength ibuprofen. Each person will have a preference according to how it works for them.
It seems there should be 7.5 mg of difference, but that is not the case. Some of the 30 mg are time released, adding to the confusion. Many prefer the 30 over the 37.5! Ok, Kids. I just re-read that non-information. Somebody help me out, here!
I have tried both the 30 and 37.5 mg. While I thought I prefered the 30mg capsules since I read that they are time release, I had to switch one month for the 37.5 and felt no difference at all both worked great.