Thank you all for your input.
It seems as if I canīt go wrong with either of them, but I think that, at least for the time beeing, I will choose the 2.5 + extension tube and see how I go along with it. Itīs less weight, less cost and I can use the extension tube (extension tube not step-up ring, I was mixing things up) on my other lenses as well.
The reason why the 2.8 micro came up is actually quite similar to fotchsī , I use a 2.8/55mm micro as my standard lens, itīs a phenomenal lens and I like it a lot, so I thought at 105mm the same would happen. Well, maybe, if funds become available ...