I have it both ways, 80 2.8 w/all three extension tubes and the 80 f4 macro w/the mag spacer. The f4 is going to be better because it is a purpose built macro. It goes to 1/2 life size alone and to 1/1 with the mag spacer. It is better but at a cost, not only monetarily but size and speed as well. If you want a true macro go with the f4, if you only occasionally want a macro, it's still a better choice but the 2.8 is lighter, faster and a whole lot cheaper. The 2.8 with three tubes takes up a bit more room in your bag than the f4 w/the spacer but are much lighter if that's important. I carry two bodies, one w/a wlf and one w/a metered finder, the macro w/spacer, a 55mm, a 150, a 210, a grip, meter, extra film and a few more pieces...yeah, it weighs a ton so I have a fold up dolly that I strap it to and roll it behind me, so weight is not that much of an issue.
Sorry for running on but my recommendation is get the macro and spacer, you will be happier.
(I think I just convinced myself to sell the 2.8 and tubes)