We haven't seen a true race comparison from anyone due to the tyres and the Mercedes' ability to destroy them at will.
However, qualifying is a straight(ish) fight and at the moment Nico is winning, which I must admit I wasn't expecting.
But, its not like Lewis is miles behind.
So far roughly speaking Hamilton has outqualified Rosberg by roughly 0.5, 1 and 0.4 seconds, Rosberg has outqualified Hamilton by 0.4, 0.2 and 0.1seconds. I'd say overall Hamilton has edged it in qualifying, so far seeing as the smallest margin he's beaten Rosberg by is the biggest margin the other way around.
Am I saying Hamilton is better, no, I said LAST YEAR and before Bahrain(after the Malaysia "orders") that when one car is faster, and is hanging on to the faster guys in front, it hurts you later in the race. Hamilton got hurt in Malaysia for holding on to the guys infront(and I still think the team genuinely underfueled him, its their calculations, not his to do), but the SAME happened at Bahrain and I said it at the time, I said before it that its happened before to many team mates and would happen to Rosberg in Hamilton's favour at some stage, and it did., Hamilton all over Schumacher a couple years back, those two fighting hurt both of their tyres, despite quite easily pulling away from Button, that stint hurt both because Hamilton was faster than Button and able to fight for that position, and that let Button cruise up and past(thats not anti Button, thats just a situation that was clear how badly they hurt each other by being in a fight for a position).
Hamilton wasn't "better" at Bahrain, Rosberg was fighting with cars from being further up the grid, and hurt his tyres, and had to do an extra stop. Without the extra stop Hamilton WOULD NOT have beaten Rosberg. In Spain Hamilton was doing more fighting and following cars for the first stint which cost him 10 seconds off the front, then the extra pitstop 22 seconds or so and thats about how much he was behind Rosberg, 70 vs 105seconds behind at the end.
With two top cars and Rosberg a good driver, poles will switch between the two, one hot lap, one tiny mistake and the other driver gets it. That isn't too surprising seeing as it happened now and then with Button and I rate Rosberg significantly ahead of Button and rate Rosberg as a better in quali than the race. They've been fairly equal this year, and its shown up more in the more difficult races which isn't surprising as Hamilton will still be getting used to settings, 3 years of experience fighting a car with dodgy tyres gives you an advantage in flicking through 40 controls and trying to get the best you can out of a set of tyres, even then...
So far for me, Bahrain/Spain are a wash, with both dropping behind the other due to having to do an extra stop due to tyres. In both races Rosberg had failures in he was outqualified and looked a touch slower in the race particularly the second race, where he was caught in traffic and dropped back further while Hamilton was on pole. That is basically what happened in Spain, having that stint in pole helped keep the Merc on pole doing slightly better with cleaner air than the team mate.
Ultimately their seasons are fairly perfectly matched all in so far, which new to a team and with a lot of new settings to pick up is impressive for Hamilton.
I think it will be swings and roundabouts, I said Rosberg would do better in qualifying than in races vs Hamilton, and I think that will show to be true over 20 races. I think as Hamilton gets to grips with the car more he'll move ahead, but we'll see. Its fairly likely the car next year will be a "fresh" start and would be a better season to compare the two drivers.
I think Alonso and Hamilton are better drivers than Vettel but I wouldn't expect either to be able to have taken Webbers drive and easily beat Vettel straight up when Vettel knows that car inside out.
Its sucked that for 2 - 3 years, fighting the guy ahead, pushing and trying to get ahead of someone relatively close to you has the general effect of actually pushing you further down the grid later in the race, than if you just sat a safe distance back and cruised around, its hurt EVERY driver on the grid at some point, several drivers many many times over, and it equalises the grid a lot, a faster driver gets punished because being faster puts him in contention with the guy ahead, and ruins his race.