Mercury 4 stroke 4hp or 3.5hp....input needed

I am going to purchase one of these two motors and need a little input if anyone can help.

On small outboards like these, how long/far does the built in gas tank (.3 gallons) run?

The 4 has an optional hook-up for a 3.5 gallon tank and the 3.5 does not.

The 3.5 actually has about 500 more rpm's.

The difference in price is almost $300.

Any input is helpful.  Thanks in advanced.