likely, you have sediment in the tank reducing your heating storage/transfer. a cleaning might restore the heater to it's original luster.
5 in the house... likely, using more or longer showers etc... a 75 will give you slightly longer hot water supply, as the burner will be a tad larger as well. also, 13 years of improvements in burners may be in your favor.
if a 50 is running out, a 75 will also. you may need an additional heater to keep up with demand.
there is also the choice of tankless, but they work best by lowering your water temp needs. output the temp water you use, not so hot it needs mixing with cold to be comfortable.

back to the 50 you currently have. it's possible the gas valve is keeping the temp lower than before, and needs adjustment or repair too...
have a contractor out to inspect the heater, and test temp accuracy, and clean the tank would be my suggestion.

 

It's probably time if it hasn't been serviced regularly for the last 13 years.

http://www.ehow.com/about_4675739_what-life-span-water-heater.html

 

Thanks very much



First off sorry for the links to the external site. I did not know they were against the rules. Sadly I believe the tank was serviced once to be cleaned quite a while ago. I could certainly bring out my plumber and have him clean it up. But I guess I am wondering if I am on borrowed time anyway. It has lasted 13 years of heavy use and I wonder if I could replace it before it dies and potentially floods are recently finished basement. My thoughts about twins vs a single large one came down to basically #1) Obviously a larger capacity (75 gallon) will allow for more hot water to be used and #2) With the newest technology lately regarding tanks I was thinking a "high recovery" unit would allow us to deal with the small time when we do not have hot water since it is unlikely we will use all 75 gallons at once. I considered the tankless but I did the numbers on the cost (unit and install) and cost of the tankless over time turned out to be more in the end than a standard tank over 10 years.

Anyway I hope this is not against the rules but the units I am looking at are as follows:
AO Smith ProMax FCG-75
Bradford White MI75S6BN

Both 75 gallon tanks. The BW is only about $100 more. It appears that the BW units are a little better rated but the AO Smith units look great too. I was just wondering if there was some big difference in the features of either one that would sway me either way.

Again, thanks so much!
Dave