As Tom states, flexibility is an advantage. He also states that in addition to being flexible, it is advantageous to have your main brewing equipment sized to your most common batch size.
I like being flexible with my mash, so I went with a 48 quart coleman cooler. This is a perfect size for my 5 gallon batches, no matter what my SG is. I have enough room to just barely hold my 11% Barley Wine or Russian Imperial Stout mashes, but it's not so big that I can't do a 1.040 or less session batch. I have no trouble holding mash temperature on the high starting gravity batches. I have a little trouble holding mash temp on session brews, because there is so much empty space. I'll drop maybe 4 F during mash on my session brews. I wrap a sleeping bag around my cooler on session brews and that allows my mash tun to hold temp.
To make 10 gallon, 15 gallon or 20 gallon batches, I have a second coleman cooler mash tun set up in series with the first one. This basically gives me the equivalent of a 24 gallon mash tun!
For boil pots, I have a ten gallon and a fifteen gallon pot. I haven't done a 15 or a 20 gallon batch yet. I have done quite a few 10 gallon batches though. On 10 gallon batches I have the option of splitting it into two 5 gallon batches between the two pots and boiling separately or boiling them all in the 15 gallon pot. The 15 gallon pot is a tight fit at the beginning of the boil for a 10 gallon batch, but it works. I wish it were a little bigger only 2 or three times a year.
Here are two photos of my mash tun set up. I was draining into the fermenter, because I was making 10 gallons of wort and was going to boil them separately with different hops. I mashed in both the top and bottom mash tuns with identical grain bills. I brought both mash tuns up to 170F, by draining out wort and boiling and adding back in. Then I drained down through both mash tuns, with all my sparge water going into the top mash tun. These photos were taken when I was measuring out the first 7.25 gallons (boil volume). I then put another fermenter down and sparged to get 7.25 gallons again. I then blended them back and forth until the gravities were identical.
You can see how flexible this system is. I can make two completely different beers at the same time. I can make a 5 or 10 gallon batch in one mash tun, but a 10 gallon batch in one mash tun has to be 1.050 or less starting gravity. I can make any starting gravity that I want to make, because of the extra room I have with 96 quarts of mash tun space.
I can mash them both at the same time. Boil them at the same time. The only things I can't do at the same time is chill and aerate. I have only one chiller and one aquarium pump.