From the standpoint of driving from dead cold the exhaust wins, and also keeps the oil cleaner (less unburned goes past the rings), you can shut off the choke sooner.
The downside is that while water can raise above thermo temperature (and can be set below that by reducing water volume), exhaust heat isn't really regulated except as to how it's designed.
I haven't figured out of way of adjusting the gas flow. Any valve will be carboned up quickly (I think), and using an inserted restriction is kind of messy and the size will change with the seasons.
