My opinion -
If you need a lens specifically for macro work, then I'd buy a used 120. And more so, I'd opt for an older 120/5.6 version, since this version, while not performing as well at infinity, will provide the best performance from 1:10 and closer. However, the 120 lens, while a true macro design, will only go to 1:4 by itself. Since you already have some extension tubes, these will work very nicely with your 120 to get down to 1:2 and possibly beyond.
Extension tubes are great in general, but limit your focusing ability, whereas a macro lens has unlimited focusing from infinity downwards.
All that said, if you've got money burning in your pocket, and desire the ultimate in macro lenses, I'd find a 135/5.6 Makro and bellows. It will focus from infinity to 1:1 with the bellows unit. Caution - be careful handling the film so as to not cut yourself - as this lens is really sharp!! (a little humor there)
Good luck.
Michael H. Cothran