Tough one, I'd take Aguero though. My reasons:
1.He would fit in our team perfectly, he's fast and has a good shot, and can score the ocassional header, something like Eto'o
2.Already adapted to La Liga.
3.He would'nt have any problems fitting in the team with Leo Messi and Milito in the squad.
4.(I know this may not be a valid point) but French players don't generally work well at Barça, the only one who was decent was Guily, but Abidal, and Henry both went downwards when they moved to Barça.
5.Aguero is already proven which means much less risk, getting Benzema would mean risking the chances of him adapting to the Spanish League. (some players find it extremley difficult to play in another league)