Are these the only two system configurations you're considering? I, through a process of morphing and meandering, ended up with a two vessel system that can be set up in a variety of configurations, including K RIMS, counterflow HERMS, single vessel, and probably others I haven't considered.
I say this not because I think you need a system that's more akin to a Transformer, but because after using counterflow HERMS I'm wondering why more people don't go that route.
What I mean by counterflow HERMS is using an external counterflow heat exchanger to transfer heat to the mash instead of a submerged coil. And when I say counterflow exchanger, I mean either a plate, pipe-in-pipe, or shell and tube. The pipe-in-pipe is commonly referred to as a counterflow, but technically all these exchangers are counterflow.
Anyhow, using this configuration allows a smaller volume of water to be used in the HLT, and I find that I get fantastic ramp times between rest steps even with just a single 1650 watt element on a 6 gallon batch. I only have about 3-4 gallons in my HLT.
Due to the fantastic heat transfer in an external heat exchanger, the wort return temp to the MLT can be very close to the temp of water in the HLT. This gives me the benefit of using the temperature of the wort returning to the kettle as the control point instead of using the HLT temp as the control point and futzing around with trying to figure out the temperature offset between the HLT and the MLT. This means I don't overshoot my rest temps whereas the reason traditional HERMS measures the HLT temp and uses an offset is to prevent overshoot.
Side note: Above I say "can be very close" because if you flow too fast on the wort side you could find a larger difference between the HLT temp and the wort return temp. Because the wort side is always throttled to prevent bed compaction, I've never found the flow rate where this happens on my system.
Other side note: I don't need sparge water as I'm a no sparge brewer. But, this configuration could still be used with a larger volume of sparge water in the HLT.
Just something to chew on.
