I think it's better to use 120mm macro for macro work for one simple reason - it's made for that.
Generally, 80mm lens is not performing its best at close distances and 120mm macro does.
This would primarily be for headshots & portraits, at least as a substitute until I can get a 150mm or 210mm.
An extension tube doesn’t magnify the lens it is attached to like a tele converter does. All it does is to move the lens further away from the film/CCD plane which makes it possible to focus closer to objects. It will still be a 80mm lens.
I understand there is no optical element that magnifies, but there is a magnification factor due to the increased distance of the lens from the sensor/film. That, and the decreased focusing distance, are the main two reasons people use extension tubes, no?
If you intend to photograph people, I wouldn't advise an extension tube.
I tried the shortest one that Hassy made on my 4/150mm because I wanted to make larger headshots: it sucked, not because of quality, but because of distortion from getting too close. I would avoid getting nearer than about 5.5ft - 6ft for heads.
In my case, a 180mm would have been better, but the only one I had owned was for a Mamiya TLR which I flogged to buy the Hassy 4/150mm... Hassy didn't offer a 180mm in those early(ish) days.
Interesting...I was thinking the 150mm would be good for headshots & portraits. I also didn't know they made a 180mm. As I mentioned above, I'm debating between the 150mm and 210mm. I'll probably rent both and compare in order to decide.