I used Canon FTb cameras before 1979, and the FD 100mm f:4 Macro was the lens I used more than any other. When the owner of the shop where I worked in Dallas required that I use an R3 for a week so that I could sell them knowledgeably, I told him that I loved the lenses and camera, but wouldn't change because there was no 100mm macro available for the R system. He showed me a brochure about the new 100mm Macro Elmar-R, and that and the dealer employee discount program (50% of list, or 10% below dealer cost) sealed my fate.

That said, I loved the 100mm FD macro and shot portraits with it, and used it with a lot of Kodachrome 25 before they took all the silver out in 1979. The lens was available with an extension tube that got you to 1:1, and went to 1:2 without it IIRC. I also always wanted the 100mm f:2 (or was it a 2.8? I think they made both and I wanted the f:2.) for available light portraits, but my budget wouldn't allow two 100mm lenses. If you can find that lens and a set of FD extension tubes, it might work well for you. I recall that it performed very well as a portrait lens. A neighbor of mine had one.

The Vivitar 90mm f:2.5 Series 1 macro had a great reputation, although I never used one, so I can't speak from personal experience. You'd be correct about the faster focusing at f:2.5.

The FD 135 f:3.5 I had was also a very fine lens, great contrast and sharpness. Those should be pretty well priced, but the close focus was about 5-6 ft IIRC, and I already had the 100 macro, so I didn't use the 135 for close-ups.

Just to throw more confusion into the mix, and at a hefty price, take a look at the Cosina Voigtlander FD mount Apo-Lanthar Macro 125mm f:2.5, which goes 1:1 with no extension tubes. It's current production, reputedly well-built, and the very few reports and photos I've seen on the net have been very positive. It has good working distance, and is fast for snappy focusing.

Hope this confuses things enough.