I'm going to agree with RossDB.
If you're just replacing your main bar get a "power" bar. The load rating should be available from the manufacturer.
Thick bars are especially for training grip strength. Might be a nice addition but not for all lifts or all the time. Other muscles might not get enough work if your grip is the limiting factor all the time.
The thickness of the bar is always a factor. Just watch some strongman competitions to see.