Llorente is definitely a better DEAL than Dzeko. But assuming they'll both be having the same wage and will cost the same transfer fee then it becomes difficult to decide... I am yet to meet anyone who regularly watches La Liga and Bundesliga's teams (excepting, BVB, Madrid, Bayern and Barca)..
For what Dzeko has been doing with City is being a super-sub.. hasnt done much as a starter. Insufficient information to make a good judgment.
I'd take the lower risk of getting Llorente for free or for very cheap... how much better can Dzeko turn out to be anyway? Its not like he can be as good as Zlatan.