OK, I know this thread doesn't deserve it, but let's try and take the question seriously.
First of all, what is "the question"? It is quite ill-defined at the moment. One version of interpreting your post would be to say "what if Nadal had been born in the same year as Federer". But as already pointed out, this is a hypothetical that is impossible to answer, because Federer certainly had some influence on Nadal's development. There are also other factors, such as the court speeds that they would have grown up with on hard-courts etc would have been different. Nadal might have quite possibly have had different idols, developed a different tennis style ... who knows. Furthermore, if your hypothesis was correct, and Nadal denied Federer some more non-clay slams, then maybe Fed would have changed his racquet earlier, eliminating much of his matchup disadvantage - and, who knows, even taking out Nadal on clay one year to win the CYGS. :O
So I'm not going to get into this, because there isn't really anything to discuss but fantasies. (It's like asking what Federer would have been like if he had been born at the same time as Laver ... can be fun, but there isn't really any way of saying anything for sure.)
So let's say we are asking whose "peak level" at the US Open was bigger, around a similar age. (There is an issue here as well - why focus on the US Open, rather than, say, outdoor HC in general? We seem to be restricting the available data. But let's stick with this.)
I am not going to do the thing of where we look at them being exactly the same age, because performances vary somewhat from year to year, and otherwise we just end up cherry-picking. So let's leave a little bit of leeway. Fed's peak level is pretty much undoubtedly 2006, one of the great tennis seasons.
Nadal's two best US Open years were 2010 and 2013. (Of course, he also won in 2017, but undoubtedly he was not at the same level.) I'm going to pick 2010 to use for the comparison, because this is closer to the age Federer was in 2006 (and that is the parameter you used), and also because 2010-2011 was the only time he ever reached the final in consecutive years. (2008-2011 he reached the semis in four consecutive years; again, the only other time he reached two consecutive semis was 2017-2018.)
So, let's compare Federer's and Nadal's levels at these events. There is another factor that we are going to have to ignore, which is that the surface speed would have changed a fair bit in the intervening years, quite possibly to Nadal's benefit. But let's leave that as it may be. Still, there are two possible questions:
a) Whose level was higher?
b) Who would have beaten whom in a final, if these two players had met with the benefit of a time machine?
Let's look at a) first. The answer is not straightforward. In the final, Nadal played Djokovic, and Federer played Roddick. Djokovic was ranked at #3, Roddick only at #9. However, Roddick's season had been beset by injuries until Wimbledon, but he had made a big resurgence in the US Open Series, winning Cincinnati. On the other hand, Djokovic had not yet become Earth's Mightiest Warrior, and had just lost in the Cincy QF - against Andy Roddick. Djokovic came off a tough SF against Roger Federer, while Roddick bear Youzhny in 4 sets (including one bagel), so he was probably fresher. Both had won a single GS at the time (two years earlier in each case), with Roddick a former USO champion playing on home soil. Roddick had two further GS finals to his name, and Djokovic one. All in all, they were probably equally worthy opponents for the final.
Fed won 6-2 4-6 7-5 6-1
Nadal won 6-4 5-7 6-4 6-2.
Pretty similar scores, but Fed's win was slightly more dominant.
Before the final, Nadal did not lose a set, whereas Fed lost one (against world #5 Blake, playing the best tennis of his life). On the other hand, Fed played two top 8 players (Blake & #7 Davydenko) before the final, whereas Nadal played one (#8 Verdasco - who on HC only ever won two 250 events in his career, and who in 2010 did not get past R3 of any hard-court masters). Blake and Davdenko also played poorly in the US Open series Masters tournaments that year. However, Blake had reached F and QF in Indian Wells/Miami, losing to Federer both times. Davydenko had won in New Haven, and made the AO QF.
Moreover, Fed won three 6-0 sets (one in the QF) and three 6-1 sets (one each in SF and F) that year, while Nadal did not win any sets at all in that manner during his run.
My conclusion is that while both played at an extremely high level, Fed's level was higher in 2006 than Nadal's in 2011 (despite Roger losing one set more).
Now, for question b), who would have won if they had played in the final? Fed had a match-up problem against Nadal (with the topspin FH to his OHBH), and he had in fact lost to Nadal in Dubai that year, in a fairly close three-set match. On the other hand, IIRC the US Open courts played nice and fast that year. Furthermore, in the 2009 AO, on a much slower outdoor HC and with Fed quite a bit beyond his 2006 peak, he still got very close to Nadal in BO5. On the 2006 USO surface, I think Fed would have beaten Nadal comfortably. On the 2010 surface, it may be a lot closer. I'll still give Roger the edge, but I am admittedly biased.
Anyway, my conclusion is that the peak form at USO for Fed and Nadal, at a similar age, is actually quite close - closer than I thought it would look, actually.
However, one thing that is clear is that Fed sustained top form at USO (and elsewhere off clay) much longer than Nadal. He won 5 in a row, and reached 6 straight finals. From 2004-2015, he lost once in R4 and once in the QF, and otherwise always made at least the SF. Nadal's record over the age-equivalent period is nowhere near this.