The size of the MLT is way more limiting than the size of the HLT, so of the choices you gave I'd suggest using the larger keggle for the MLT and 10 gal pot for the HLT. The suggestion by milldoggy is also a good one. Whether the 10 gal HLT is big enough for you depends entirely on your system and process. If you batch sparge it should be big enough for 10 gal batches. The only time it wouldn't be is if you used a thick mash on a low gravity beer, but in that case topping off the boil with water isn't going to hurt your efficiency. If you fly sparge and the 10 gal HLT isn't big enough, the solution (other than buying a larger vessel) is to use the BK for first part of the sparge water, and drain the first runnings into a spare container (bucket, corny keg, etc). Once the BK is empty it can then hold the runnings and sparging/brewing continues as normal.