This is a tough call, because it does indeed get hot enough in Las Vegas to cause the same problems as Phoenix and Texas (there just aren't enough LEAFs there to register a trend).
On the buy decision side - honestly, range will not be a problem for you, no matter how bad the battery degrades. When gasoline goes up, you just laugh. Also, you'll be the envy of all your friends when you click on the air conditioning from your phone or Internet (or the car's timer).
Negative side - parked outside, always exposed to the heat (although some garages are hotter then the ambient air). I probably would NOT get any electric car without a place to charge at home.
I would get the LEAF, and just know that for long term ownership, you'll probably replace that battery sooner than somebody not in brutal heat. For short term ownership, you'll likely take a good hit on value with a seriously degraded battery.
My personal note: don't let somebody convince you that Nissan is going to "take care" of any battery issue in the future, unless they are backing that with a bond for the value of the battery. Nissan has recently said as recently as a few days ago that 30% + degradation in one year is "normal". They may indeed fix things, but things haven't start out well for a VERY serious, financially and usability significant problem.
Honestly, until I saw a plan and action to fix the cars (that really works, and not a patch on a bigger problem), I only recommend leasing the LEAF in hot climates.
