I was saying something similar
earlier in this thread.
Obviously Dwight is the better of the two, but Deron seems like he'd be a better fit in our current system... He'd be plug-n-play, whereas you'd probably have to rebuild from the ground up if you only added Howard.
I'm not going to make any arguments against getting both guys, but there are definitely some advantages to getting only one guy, especially if that one guy is Deron Williams:
1) You don't have to trade or amnesty a very productive Shawn Marion.
2) If you're not getting Dwight, then you don't have to trade or amnesty Brendan Haywood, who -- despite what some people think -- is totally worth what he's getting paid in this league. Plus you could pick up the team option on Brandan Wright and lock up the center position for a mere $9.3mil (Mahinmi aside), which is about half of what you'd have to pay for Dwight Howard alone.
3) With the rest of that Dwight money, you could re-sign guys like Jason Terry, Vince Carter, Delonte West, Ian Mahinmi or go after any number of free agents on the market...