Here is my point. in stock form 305s ARENT fast, but, they can run very strong. as far as them being more efficiant, In stock form, I dont think so.
Now with that being said, it doesnt mean I hate 305s, Ive built a couple fairly quick ones my self
But fact is dollar for dollar a 350 will make more power. hell just a a stock rebuild kit is almost 100.00 cheaper for a 350 than a 305.
As far as MPG, depends how its used,if both engines have to work hard all the time, Id say the 350 could get as much MPG as the 305. for light duty, everyday driving, sure, the 305 might do a little better.