Well what do you mean by benefits?
I can tell you that they both have the same base HP and speed. Dustox's defensive stats are the same as Beautifly's attacking stats.
For example; Beautifly's Sp. Att = Dustox's Sp. Def
In my opinion though, both are pretty boring pokemon and they are nothing more than the mandatory bug pokemon evolution that we get in every version. If you have to pick one, Beautifly has the more attacking stats so capture something that adds to its Sp. Attack, as for Dustox go for Sp. Defence raising natures.
Personally I'd prefer a Dustox, they learn psychic attacks and also learns toxic by level up. Both learn Silver Wind which is meh, 5PP isn't anything much and after that you have Mega Drain which is also pretty boring in my eyes. 10PP for a 40 Base Power? You could always teach it Giga Drain I suppose, but even then thats only 5PP.
If you must get one, try for Dustox, but I can't say about personalty values because all the ones I've captured have been Beautifly and that's before I discovered EV's and IV's etc.
Just what I think anyways.