HELP - 40 or 50 gallon tank

I'm about to order my hot water electric tank. My only real daily use is a shower. I have following concerns about having enough hot water for a shower, that may cool too fast.

Per Rheme's training manual (I am buying Kenmore):

1. 40 gallon tank (38 short tank) has a calculated hot water for a shower for 12 minutes. It has first hour delivery of 45, recovery of 25 (if using a 3 gallons per minute shower head, hot water available is 28 gallons at 120 degrees, and 36 gallons available at 10 degrees of themostat).

2. 50 gallon (would be medium tank) is 15 minutes for a shower. It has first hour delivery of 59, recovery of 25.

Then it says - confusingly - on the usage to determine what size tank to buy: Gallons needed are 24 gallons (for a shower 20 gallons, and hair washing 4 gallons).

The tanks are similar except the 38 gallon has 2.5 insulation (R factor 20) and the 50 gallon has 2 inches (R factor 16).

Should I just get the 50 gallon tank? My concern is the ceiling is a bit low and removing, replacing an anode rod may be difficult? The 50 gallon is medium at 48 inches and the ceiling is about 64 inches.

A couple of things...you never can use all of the hot water in the tank before it starts to get diluted and cool off. It helps that you draw it from the top, and there is a heating element up there on most of them to maximize how much you can draw, and that you add the water towards the bottom via the dip tube, but you'll never get 40-gallons of same temp, hot water out of a 40-gallon tank. Rough rule, maybe 75-80% of its volume. Second, the do make segmented, bendable anode rods specifically for situations where you don't have enough room to use a straight one. Running the tank hotter and using a tempering valve makes the tank 'look' bigger, but the bigger the temperature difference with the ambient, the higher the standby losses. I'm sure there have been studies on quantifying that, but I've not looked.

The surface area of a 50g tank may not be 25% higher than a 40g tank, so given the same levels of insulation, the difference in standby losses is probably small.

Thanks, jad. In summary I am hearing that the 140 water tank temp and a low flow shower head (with a switch to turn off when soaping up) will give me enough hot water. This keeps me with the better insulated tank (2.5 inches and R value 20) and a thicker glass lining. Along with insulating the pipes.