First of all, you should try out a macro lens for your uses, before buying one.
All macro lenses will be slower focusing than your 70-200mm. This is by design... they have what's called "long throw" focus, which emphasizes precision over speed. Maybe they are fast enough for you, maybe not. Having USM and a Focus Limiter helps both the Canon 100mm macros focus faster than some other macro lenses, but they are still not speed demons. You should try before you buy, if at all possible.
I also am not a big fan of macro lenses for portraiture. They are typically too sharp, showing every little flaw of the subject (and no one is perfect except for 17 year olds after a 2 hour make-up session). I'd much rather use an 85/1.8, 100/2 or 135/2 for portrait work. They're all fast focusing, too. The 85/1.2L is a "wedding photographer's dream lens", although like the macros, it's not super fast focusing.
For the level of magnification needed for ring shots, cake details, center pieces, and other typical wedding close-ups your 24-70L should work great. If you wanted a little higher magnification out of it, just add a 12mm or 20mm macro extension tube to the 24-70mm. (The Kenko macro tube set for Canon is a great deal right now... price has come down to $109. A year ago they cost $200... And the individual Canon tubes are selling for $84 for the 12mm and $145 for the 25mm).
Not sayin' that the 100L wouldn't work fine for your purposes... you may like it. Personally I use the 100mm non-IS version a lot for macro work... but not so much for non-macro purposes.
So, I'm just pointing out some other options.