No, you don’t need both, as they are two completely different features. The upfitter module is the VSIM, and deals with various digital outputs such as locks, lighting, wigwags, etc.
You would only want the aux switches, which come with the auxiliary fuse/relay box already set up in the engine compartment, with easily configurable outputs in the engine bay, under the dash, and at the bumper, so you have minimal wiring to run to any accessory anywhere without having to pull wires through the firewall. You can program how each switch behaves at any time- battery power, ignition power, remember last state on restart, etc. This is one option that anyone should add for it’s price, even if they don’t think they’ll ever use it, because it’s such a pain and expensive to add in full factory form as an afterthought.