You are better off with the flash.


Optically, the differences between the EF 100/2.8 macro and the EF 100/2.8L macro IS are negligible. In fact, I would even say that the latter has more lateral CA than the former. The only real benefit is that the L has IS, but even this is of limited use at 1:1, with only about 1-1.5 stops advantage in practice. In my opinion, the IS really only helps in terms of keeping the viewfinder steady for composition and slightly more accurate focus placement. However, it does not help with respect to movement parallel to the optical axis. Yes, the L is weather-sealed, but I don't think this is terribly significant for you.


The ring flash, however, will help tremendously, since it is only through the addition of sufficient light that you will be able to attain shutter speeds fast enough to permit sharp photos at high magnification. I do strongly advise you to consider the twin flash, though; although significantly more expensive than the ring, it offers far more lighting flexibility not just in terms of position, but also in the use of modifiers. The ring flash was designed primarily for medical and dental applications.


Flash lamps last quite a long while. They don't really lose output power over their lifetime, AFAIK, but if the lamp fails it fails completely. It is more likely that an internal electronic component (e.g. capacitor or circuit) fails before the lamp itself does. And so in general, if it hasn't failed yet, you can't really know anything about the internal electronics without opening it up.