This flip twixt Vegas and Phoenix is interesting. It’s likely the heat island affects Las Vegas more than Phoenix since we’ve seen it in Phoenix for many, many years, but Vegas has expanded exponentially in just the last 3-5 years.
They had some evening cooling, but that’s probably over...look for Vegas to take the heat title in future.
The Mojave also has different weather patterns than the Sonoran. We are subject to air flows (or lack thereof) off the Pacific - over 3 mountain ranges - from either Southern California or sometimes from the Bay Area (NW). The flows into southern AZ from Mexico are less obstructed and can come from both the Sea Of Cortez and overland from the Gulf. There is more moisture in Southern Az as a result. That may have impacted the weather more this year - we got no monsoons at all - so even though we are much higher in altitude we were hotter. If ElNino comes back like last year when we had a cool, wet summer (for us), things will go back to what we more normally see. Regardless of growth, the heat island in Vegas is still quite a bit smaller than Phoenix.