Conventional wisdom would indicate that a double boiler will be more temperature stable and consistent than a heat exchange system. That's the problem with conventional wisdom, it seldom takes all the important factors into consideration. Lets examine the two systems you are looking at, shall we?...
Thanks a lot, Mike! Now I want a Fiorenzato Bricoletta! That's a really nice machine! :)
If you use the continuous flow button, the bypass toggle switch, or you have a semiautomatic, you don't even get this electronic pulse. Rather, you get a full-on 9 bar blast right from the start.
In terms of sheer efficiency, double boilers hand heat exchangers their asses.
A couple of thoughts, totally skirting the HX vs DB question.
What is the service support for Faema and La Marzocco in your area? Faema support in some areas (including ours) is spotty, and since parts are distributed through the La Cimbali network things can be really difficult.
Pass on the autosteam option. You can do better with a traditional wand. Plus, the autowands give you many more moving parts and controls to fail. Maintenance will be more expensive for a machine with an auto wand (unless it just sits there unused).
How old is the Linea?
On the PID question, I submit for your consideration the Competition Aurelia. HX + PID to good effect. (OK, maybe I won't skirt the hX vs DB thing...)
Me? I like hX machines, but I'd go for the Linea.
Jeff, I take it you have not met or worked with my best new staff "Aurelia" There is a distinct reason these machines were chosen for the WBC.
Have mentioned it here previously, but here goes again. I've had the pleasure of using two La Marzocco machines, a 3 group Linea and a 2 group FB/80. The Linea wasn't my fave by any means. Each of the 3 groups had varying water flow rates as well as lousy temperature stability. Could've been the refurb job that was done on it or maybe I just didn't understand the machine all that well. BUT did work on it daily for 7-8 months and was hard pressed to ever get anything I'd call great from it regardless of what tinkering was done. Now the FB/80 was a great machine. Much more refined than the Linea and much better consistency based on the time I spent on it. Really like the fit/finish of the LMs.
My daily driver is a 2 group Aurelia and I love using it. Now it does have some plastic on it, but the machine has a nice look to it IMO and the fit/finish is quite nice. Under the hood it's all business and I really like the parts layout and fairly good access to everything.
I do agree on skipping the autowand gadgets. I do use ours for heating cider and that sort of thing. Will say it works great for things of that nature because the temperature can be programmed and it hits that point every single time. But I wouldn't waste time trying to texture milk with it.
Don't know about any sort of flushing technique because I've never had to do any. Anytime I've checked the group temp. it is always in the 199-200 range, which is where I choose to extract at. Never seen it register more than 1 degree higher/lower than those numbers regardless of whether I'm extracting 1 double each minute or 1 each hour. It is always predictable and simply dead-on.