I think that Nani is the better player overall, but I think Rooney is often played in a role where he is not comfortable, when relied upon to create chances and it makes people think he's in bad form.
A lot of people seem to expect Rooney to be able to do absolutely everything, because they think he can, well I don't think he can at all. Don't get me wrong, he is great at many things, such as his work-rate, his long range passing accuracy, his discipline (positional), fitness, heading and his goalscoring record is very good.
What I don't think he is 'great' at, is his touch, his short passing, his creativity and his dribbling. However some people seem to think that he has these things in abundance and throw their toys out their pram every time he has a bad touch thinking that this isn't normal for him. In similar fashion to Hernandez, I think he gets so much unfair criticism when he is being asked to perform a role in which he is lacking in certain elements needed in this role. A lot of the time, the team can attack in the wide area and Rooney can easily adhere to this, but in tougher situations where we're restricted in those areas, it's down to Rooney in the middle to create chances, which is in my opinion an absurd responsibility for Rooney to have for reasons I have stated above. It also creates an environment where people think Rooney is having a poor game and freely air their frustrations at him. If he were used more effectively I think what some perceive as peaks and troughs in form would certainly level out. On the flip side, in the role he plays he is suited to helping out defensively and getting the ball wide - but not fully qualified to play this role in the tougher games where wide play might not always be a viable option, in my opinion of course.
In games where he can be truly effective though, I think he can be more influential than Nani for sure, although it could be argued that the task Rooney does - influential it may be - is slightly easier than the job Nani does i.e. beating players and more critical decision making. I think Nani's strengths are his dribbling, skills, link-up play and shooting. To me Nani is clearly the type of player who can play in any league with these skills. I'm not at all confident that Rooney could cope in a more technical league such as the Spanish league. I think he would struggle big time. Of course that is pure speculation but based on the traits and weaknesses of him and the properties of that league I think it's a fair opinion.
To conclude, I think that Rooney plays a more influential role for us, but he's not as great in that particular role as we think or expect. I'd rather see him playing in a more advanced position, or maybe not even more advanced, just with less responsibility for creating chances in tough games. In that position I think there would be no problem with his 'form'. With Nani, I think he is much more talented technically and as a winger he does exceptionally well for us. His limits know no bounds as he is not limited by technical ability, and for that reason I voted that Nani will be better in two years time, as he has all the talent and all the potential to improve even more.