My MRC 2800 runs 10.5 volts DC.It is made for slow starting. An 18 volt is from a starter set. Mostlly a toy. So it's more depends on a brand than a voltage. The better engine do operate on the lower voltages. DCC is another story. http://www.modelrec.com/search/adva...?Category=TRAIN CONTROLS&SubCategory=DC Power
I see your point now.Some are 18. Read some manuals and see what the other members are using. One consideration is how big of a layout you want to power.
On the second look. see the Tech 4 models they are 14.5 volts. I looked at the 2600 high power and the 18 volt mode was for LGB trains.Stay 16 or less. I always liked the dual control models.