Which to install 391 rearend gear or lower?

I think it really depends on how your engine is built and what size tire you have. 3:91's aren't enough for my car but for some 3:55's are still way to steep. 3:91's will be pushing it for hwy use though unless you have tall tires.
Engine bored 30 over
Cam 501 lift
Tire size P245 60 R14