The kegs would make a larger mash/lauter tun than the cooler. A little more work to convert the keg versus the cooler, but it's really up to you. The coolers might do a little bit better job and maintaining heat, but that probably depends on how you insulate your HLT and MLT. Either way you can't go wrong. If it were me, I'd probably sell the coolers and use the money to convert the HLT/MLT.