The differences depend on the tartan you're after I would think... Order a small swatch of the desired tartan from each mill and judge for yourself. It's the only way I can think of to get the closest match. You can also then use the winning swatch to make a striking cockade for your balmoral or a dazzling hankie for your top pocket.

Lochcarron Swatches

House of Edgar Swatches

Note HoE appear to do two ranges of heavyweight, Clan Heavyweight and Nevis Heavyweight. Not sure what the difference is.