Maybe someone who has worked at both can chime in.
It's not that easy to say one is harder than the other. Some people go from Class D DVT to Class B PHX and struggle, since PHX is much more proceduralized. Others go from PHX to DVT and struggle since DVT is much more seat-of-your-pants controlling.
There is much more to determining facility complexity than mere traffic count. Is DVT complex airspace? What does their mix of traffic look like? Intersecting runways? Mountainous terrain? Do they get much military traffic? How about Air Carriers? Are they Class B, C, or D airspace? What is their Itinerant count vs. local pattern traffic? Every single one of these criteria counts for something when determining how hard a facility is, and "hard" is a very subjective definition in this context.
Will you get your ass kicked at DVT almost every day, probably, but so will the guys at PHX. PHX has more concrete they need to move around which immediately makes their job harder than DVT. The fine folks at DVT have to deal with 97% of their traffic on a "V" tag all day long, having to think of creative ways to move them in an out. That makes DVT more difficult in that sense. Different facilities, different definitions of "hard".