It's not that simple, By cutting off the Long 15. it will cut off NCR's most vital trade route, or as Ulysses worded it, cut the bear's throat. As for Dry Well's, beyond it destroying what used to be the Twisted Hair's old home before being betrayed by Caesar, I don't know what if any real impact that would have on the Legion's future.
Ulysses never intended to nuke Dry Wells.
Ulysses Plan
Step One: Launch nuke at Long 15, this cuts NCR Supply Lines, the NCR forces at Hoover Dam will lose to attrition
Step Two: The Legion takes the Mojave. The Legion then expands to the west, then Legion will collapse in on itself. Both the NCR and the Legion will be gone.
Step Three: ???
Step Four: Profit