I would consider a third alternative:
const string quote = "\"";
string whatever = "! U1 setvar " +
quote + "power.dtr_power_off" + quote + " " +
quote + "off" + quote;
Or a fourth:
static string Quote(string x)
{
const string quote = "\"";
return quote + x + quote;
}
...
string whatever = "! U1 setvar " + Quote("power.dtr_power_off") + " " + Quote("off");
All these have pros and cons; none is clearly the best.