SLC is much closer to Moab than LV, so if it's Moab you want to see then SLC is better than LV. You can fly to Moab and Grand Junction as well. Sometimes flying into Grand Junction isn't much more than flying into SLC, so check into it. Flying into Moab is more expensive, but highly scenic, especially if flying from somewhere like Page.
If you wanted to do a one way trip of all the parks starting in LV and to Moab, you could likely do it dropping off the rental in Grand Junction. Grand Junction is less than 1.5 hrs from Moab.