I normally brew 12 gallons and I have two 20 gal Blichmanns for MT and Boil. I have the 15 gallon Morebeer for a HLT. 5-6 gallon batches don't hit the thermometer in the BK. Depending on grain bill sometimes in the MT.
I prefer the Blichmann kettles -- I like how the lids hang on the handles for quick storage. The welds for the handle broke on my Morebeer lid. I also like the tilting "Brewmometers" and the sight gauges. Those make it faster/easier to hit volumes. On the 20 gallon kettles the sight glasses start around 3.5 gallons.
They are lighter and easier to move around but at the end of a batch I can see a "print" of the metal parts of my burner so the heat distribution is better on the Morebeer with the thicker bottom. I don't seem to scorch anything, more like an outline of baked on proteins. I don't have actual data to support, but it seems my boil-off rate is less than when I only had the Morebeer kettle. For 10-12 gallon batches, it is harder to boil over in the bigger kettles but still possible.
I got about a 10% efficiency boost going from a cooler and CPVC manifold to the Blichmann false bottom especially on bigger beers (from the high 60's to the high 70's). The pick up tube works better for me than the Morebeer silicone and copper contraption too, though both are adequate.
If I was going to be doing mostly 5-6 gallon batches, I'd probably plug the original thermometer hole and punch a new hole to move the thermometer down -- especially in the mash tun.