I'm not voting, but I will give some information. It may or may not help.
I have both (sort of). I have the original EF 100mm f/2.8 macro non-USM (not the current USM version) and the L.
Usually when I do macro, I use a tripod and/or flash (in manual mode). So IMO the IS is less useful/noticeable/needed under those conditions. The IS is not used by me as much for macro. I do use IS sometimes for macro and it is better than not having IS. I use the the IS more for other types of photography with the lens. IQ between the two lenses is real close (the L is a bit better).
So why did I upgrade?
The original 100 macro extends when focusing (this didn't bother me - some people it bothers), couldn't take a tripod collar (I wanted this), and AF is slowish/noisy/non-full-time manual override (in macro for me a non-issue, but for non-macro this can be an issue).
The USM version came out and remedied these issues. That wasn't enough to get me to upgrade. I just mainly relegated the 100 macro to just macro, and used other lenses for other types of photography.
The 100L added IS and weather sealing (plus had all the improvements of the USM). I'm getting older, IS is a benefit for me. Now it was worth the upgrade to me. I now use the 100L for mainly macro, but also use it for other types of photography (seldom did this with the non-USM-Non-L).
If I would have had the USM I might not have upgraded. I probably would have though. I like macro and I like other types of photography. The L and IS was a luxury that at the time I wanted, and could afford.
Maybe there is something you can use in there. If money is tight and you can live with the limitations of the USM (much less than I had/have with the original version) then it is a great deal for a great lens. If you think that you will really get into macro plus use the lens for other things beyond macro, and can see many instances where the IS (*IMO the only benefit) will come in handy for you - go for the L.
* Yes, there is weather resistance. I still protect my equipment from the inclement weather with covers. It is weather resistant not proof, so not much of a benefit (to ME).
I didn't vote because each is a great lens, and not everyone needs or wants to spent the extra money on the L - I did!