305 vs 350 - 1991 firebird

I am planning on buying a 91 firebird for cheap so i can simply replace the engine and whatever it needs as i see this is much cheaper then buying expensive, and replacing stuff anyway.

Simple question: 305 vs 350

Most say go for the 350 it gives way more power and is worth it. I personally want the car for school/work purposes so I'd rather go for the better gas milage

What's the gas milage rated at for a 305 with tbi or tpi?
What about for a 350 with tbi or tpi?

I wanna go with a 5speed, its better on gas, and can be quicker/more fun at times. If i decided for whatever reason on the 350, does it require a new computer, intake, exhaust, etc? If so i would probably scratch that idea.

The life expectancy would be much greater with which one?

Well, my friend has a 305 and he gets between 25-30 miles per gallon on the freeway and he has a 305 with a Carb. on an 86 TA, and he has that thing doin 14's right now with the stock bottom end...

my brothers 305 in his 91 camaro gets horrible gas mileage, it my moms lt1 probobly gets better gas mileage then it does... and the 305 was recently rebuilt and the fuel filter was replaced probobly a few hundered miles ago

does that 91, 305 run a carb? or a fuel injection?
standard or automatic?

i wouldn't imagine a 305 being terrible on a fuel injected 5 speed, rebuilt, and properly tuned up. I was hoping for 17-20 city. I could be wrong though...