It's recommended price on release for the 150OS is going to be $1600 or ~£1000
That's into 180L territory, but the RRP for the 'old' 150mm was £730, and I never saw anywhere selling it for that much.
The 100L is another superb lens. Probably sharper than the 150mm wide open (maybe relevant for general use, but you won't be opening wider then f/5.6ish for macro), and with an excellent IS system. The IS loses effectiveness at higher magnification, and a tripod or a flash setup is probably more useful, but a stabilised viewfinder might be very useful for fine focussing handheld.
I expect the 100L is also lighter (thanks to a special plastic body instead of metal, and being a smaller lens), and it probably focuses quicker.
The 100L arguably has more use as a portrait lens and general purpose medium telephoto, as well as being easier to handhold, but the 150mm is cheaper and has a better working distance (and will produce a more diffuse background blur).
You've just got to weigh up which strengths are more useful to you.
I bought mine specifically for insect macro, so the working distance was key. If you look
second hand, you can also pick up a good deal on the also excellent sigma 180mm macro. It's even bigger and heavier, and a bit slower (f/3.5), so even less useful as a general purpose lens, but you get even more working distance and even more diffuse backgrounds. It does move even further out of hand-holdable territory though. The 180mm is now discontinued in favour of the 150mm, but this is only due to it being less popular, it has the same great optical performance.
Other users say the 150mm/180mm also take a 1.4x TC or 2x TC very well (especially if it's for macro since you'll be at small apertures).
This guy even got very usable results from stacking 1.4x and 2x TC's on the 180mm, a mark of how well these lenses perform
http://www.juzaforum.com/forum-en/viewtopic.php?f=3&t=7