Sprinkler heads: No don't worry about them, either tractor can mow them down or snap them right off without any problem. But seriously, if they are the pop up variety and if they are firmly in place and the ground is stable around them then you should not have too much trouble with them for either tractor. If the ground is soft or the head is poking up then they could shift under the weight of either tractor and be damaged.
Septic: if you have clay tiles in your field then I would strongly favor a lighter tractor, if you have plastic pipes and the field is well built, either tractor should be fine. As for driving over the tank itself, again it would depend on the age and construction and depth and soil. The newer tanks (past 15 years) seem to be better/stonger than the older tanks. There was a long thread not too long about about septic. I've not had any problems, but my system is only 10 years old. I wouldn't see any problem with the TC24, I'd be mildly cautious with the TC33, if it was simply mowing across it then I wouldn't worry, but I doubt I'd want to park on top of it.
Soil conditions dictate a lot of the problems.