The rackers calc tells us you can mash 32 lbs of grain in 8 gallons of water with a resultant mash volume of 10.56 gallons, or less than the 11 gallon 44 qt kettle.
That's a big 5 gallon batch!
Or 20 lbs of grain in 9 gallons of water results in a mash volume of 10.60 gallons, perhaps more realistic....
http://www.rackers.org/calcs.shtml
In the event that you may want / need to do a small sparge, say a gallon, even a cold water sparge would suffice negating the need for a another vessel.
The only possible advantage to the larger kettle is increased headroom while coming to a boil, with a 15 gallon kettle, with the proper wattage you could walk away and not fear boilover, or more likely a little wort splashing out the over the kettle rim. Even with the smaller kettle running proper wattage, boilover is not really a concern. With conservative wattage, an e-kettle approaches boil not as aggressively IME, and boil over threat is not like a propane burner IME.
The 44qt is lighter, easier to clean and sized nicely for 5 gal batches.