A reasonable alternative is the 150 Sonnar with Proxar lenses. The results are superb. The 150 is abundant, and wonderful. You lose NOTHING with the Proxars, and are far more convenient than extension tubes.

You gain convenience: being a little further from the subject, you can get out of your own shadow.

The 120 is a wonderful lens, but was intended for high contrast reproduction of fine detail up close. In the field, I've always felt it was a little short.

For a couple years, the 135 makro Planar was available with a focussing extension tube, and was the COOLEST lens you could imagine for portraiture, fashion, and so on. But it was slow. Still, you might find one laying around.

good luck