Sorry, da gino, I deleted my original post as I thought I had gone off topic a little

. Next time I will just edit it!
I agree that cost is non-trivial and important.
Endo: Why I posted originally was to mention that heatup time should not be used as a benefit for single boiler espresso machines (when considering like-for-like). For example, a single (or double) boiler E61 would take at least as long to reach brewing temp as an HX E61 machine, if not a little longer. I also don't agree with the comment about less complexity, nor less energy, compared to a double boiler (in which the steam boiler can be turned off). However, all the other points are worth considering

.
Edit: There is, however, a benefit to having a "second, but cheaper" complete machine dedicated to steaming: you have a backup brew group!
All the best,
Martin