I have the 245/70R16 myself and they work very well for everything I've tried in them except heavy mud and differ off-road obstacles. The extra centimeter or so in diameter of the 245/75R16 makes a big difference off-road BUT as stated, you probably won't clear your body and suspension unless you have a lift.
So, in summary:
- 70s for all around use and light off-road (no issues with clearance)
- 75s for more off-road use and lifted (will have clearance issues without a lift).
There are other sizes that some folks like that fall into the gray area of hight/width/etc, but since you just asked about those

Dan
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2005 Silver CRD Limited

245/75R16 GoodYear Duratracs
Fumoto drain | ProVent CCV Filter
Stanadyne FM100 filter | Cummins fuel pump
GDE Eco | SEGR | BoulderBars | FrankenLift | Frankenskids