That's a good question. I view Vegas as more notable as it really does appear to be an isolated region. Nevada is more desolate overall than Arizona and likewise I feel Phoenix has less of that desert city feel being surrounded by other cities.
There really wasn't much south of the rim in az.. You had tuscon and the valley.. Scottsdale, Mesa, Tempe, Phoenix, Glendale sit on a natural aquifer, along with the salt River.
Vbut really the population here was doa before AC,and lake please t and other reservoirs were created.
I mean it bloddy doesn't rain here, nor snow. Everything here can kill you.. It's AU in the USA... Granted vegas is seemingly isolated, but i reality it's not, Henderson, vegas are just over the boarder from Kingman and not far from the i40... Obviouslly tourisim is big as is gambling, but the area has been trying to also diversify.
Same with Phoenix that also has casinos, but is actually a major city (looking at it as McDowell to white tank Mt and South Mountain north to Anthem, it's a very large area.. Something near 160 km across and about 110 north south.
But water.. Heh.. This place is sand and cactus and if you go a little further south it's even more desolate.
Hoover damn was critical for Vegas and Arizona, but if I was gonna say a major city in a place one shouldn't exist.. I would say Phoenix and both Vegas qualify.
Also Vegas is an absolute textbook case on waste of resources