Honestly, if it's not too late to take it back, I would. Especially if you plan on batch sparging, because having the mash tun bigger than you need is not really an issue for batch spargers, and batch spargers need larger mashtuns because you need room for grains + several gallons of water.
As others have said, there will come a time when you want to be able to mash more than the 13 lbs or so that you can fit in that 5 gal--whether it's for a bigger beer, or doing a 10 gal batch with a brew buddy or whatever.
I use an old 8.5 gal rectagular as my regular MLT, but I can jury rig my manifold in a 12 gal for larger grainbills if needed (like for a 10 gal batch). The 8.5 gal cooler allows me to batch sparge grainbills up to about 18 lbs.