Is there that much difference between the two? I'm looking at used 100mm 2.8 macro lenses and some have it and some don't. Would it make much of a difference in price?
For the macro I would say you will save a bundle by not having the USM, and if your main purpose is macro photography, you will be using manual focus anyway so I would say get the non USM version. I had a sigma 105mm VERY SLOW focusing but did not bother me because I used it in manual mode.
Of course, the 100mm f/2.8 USM macro is also a fantastic medium telephoto prime, so depending on whether you'll be sticking to only macro usage or using it for other purposes will determine whether you need USM.
the difference isn't only in the USM implementation. the USM version has inner focusing so the length doesn't change and the front ring does not rotate (important for not spooking the things you're shooting). the newer version also has full-time manual focus. canon says they are of completely different designs.
So how fast is the 100/2.8 USM compared to regular USM lenses? Is it usable for sports for example? I could imagine the close focusing ability might hamper its speed in the normal range (even when using the focus preset).
I'm also debating saving some money by going for the old one. I'm leaning towards the USM version though.
With the slower focus you'll probably only use this lens as a macro and/or complain the AF when used as a telephoto. Whereas if you buy the USM version, even if you don't plan to use it as a telephoto, you'll still have that as a very viable option. Also, if you sell the lens at a later date, you'll recover more $$$ on the USM version.
This is not related to the focussing speed, one of the more impt difference between the 100 macro USM and non-USM is there is extension of the front element for the non-USM version. If you're shooting lots of serious macros, you might want to get the non-extending USM version.
I have, and do love my non-USM version though. It has been appreciating in prices over these few years.
