My wife, both sons, and I have all used a .243 very successfully for both deer and antelope. Always we have used 100 gr, just because I felt happier using a slightly heavier bullet.

Certainly, as has been stated, shot placement is of prime importance. However, I've also killed many deer with .270, 30.06, and 7mm mag with bullets ranging from 130 to 180 gr. With a well place double lung shot, the deer have consistently gone further after being shot with a .243 than with any of the others.

Bottom line - I would certainly use 100 gr rather than a lighter bullet.