Older Arduino's used 5v but newer units are using 3.3v more often. I have some Pro Mini modules that have a toggle switch to pick3.3v or 5v. But when used in 5v mode they are non-standard and harder to use without customizing some things in software. Basically, 3.3v is becoming standard with the AT micro controllers too.
Then there's the another option... EPS8266. It incorporates a decent processor with WiFi capabilities in a package the size of a smaller Arduino, inexpensive, and works with Arduino code/IDE, too. If we could use one of these instead of a PIC, we could have WiFi capable X10 modules (though I'm not sure the benefit of that since X10 communicates over PLC).