They are slightly different to the point where mags won't interchange.
Partially true. All 10 or 15-round factory mags work in either, but newer 16-rounders won't fit in the older non-B 75. That can be remedied by opening up the top of the mag channel in the 75 -- something that a gunsmith can do.
Some after-market mags work fine, as do some Tanfoglio mags. You can't buy sight-unseen; you need to try them. Most Mec-Gar mags probably won't work, and Mec-Gar now makes the factory mags.
The original 75s generally had great triggers. Some of the late 75s look and act like a 75B, except they don't have the firing pin block; those models will generally accept the newer magazines.
The 75 has evolved with a number of modifications over the years, with subtle changes in the barrel, magazine brake, sights (and related dovetails), etc. The newer models come with a less pleasant trigger, but that smooths out with 300 or so rounds.