If you're renting, go for either the 300 or 400 L, don't bother with zoom. Yes, there will be instances when the action gets too close ... but lacrosse game action doesn't tend to stay in one place for very long and most of the field is going to be far away from you just about anywhere you setup. I've shot lacrosse with the 100-400 IS L and the 300 and 400 L lenses and have to say the photos from the 300 and 400 are hands down far better quality than from the zoom. The fast lenses, of course, make it much easier to keep your shutter speed at 1000th or faster, which is ideal for lacrosse, but the sharpness of the lens and shallow depth of field are something you can't duplicate with a zoom lens, even at high ISO values.
Just as a head's up though, the focusing speed on the 5D is pretty slow and can make it difficult to keep up with the speed of lacrosse action. I'm sure you'll get plenty of good shots, but it might be challenging to get good focus if you shoot from the end lines, since the players would be running at or away from you most of the time. Setting up on the sidelines will provide more opportunities for you where the action isn't moving at or away from you so you're focus has a bit more time to keep up.
Hope this helps.
Have fun.