WashU has a good medical school, which has nothing to do with undergraduate premed. It would be foolish to argue that a good medical school makes the undergraduate premed experience better. Imo, there is little difference between premed at top private schools.
Is Northwestern/WashU worth the extra money if I plan on going for med school?
Is the possibility of distraction at UIUC a major factor to consider in maintaining my grades?
There is a possibility for anything to happen; I wouldn't worry about it.
It seems like Northwestern has a safety net--if i end up not liking premed, I can get a marketable degree from Nwestern whereas WashU I can't--is this true?
No, but NU is much more prestigious/recruited compared to WashU.
Where should I go?
Imo, NU or UIUC