Downtown Summerlin is huge! I think once all of the stores are open, it will be impossible to get through the whole place in one day! The mall itself is really pretty. Being from the Midwest where there are no outdoor malls, I love the open air. Although, it will be rough in the Summer.
Of course there are tons of stores. My favorite is Nordstrom Rack - it's much bigger and has a better selection than the one in Henderson. There are also lots of restaurants to choose from - anything from a quick bite to eat to places with a waitstaff.
I still prefer going to Town Square just out of sheer convenience. Downtown Summerlin is literally straight across the valley from my house, so it takes at least 30 minutes to get there. Other than that, it's a great place to shop!