Well I've already made my post in this thread, giving the nod to Queen, and I stick with that.
That being said, though, that I for one can - at least partially - understand the heated arguments.
We obviously have two great bands here. Now by the difference in styles alone, these two bands will probably have very different groups of fans. While I see Queen as a more "Mainstream"-compatible band, since their music isn't what one could consider too "hard" or too "metal", and some of their (especially the 80ies) stuff is quite close to "Pop" music, with the massive use of synthie sounds in that period of time, additionally to the still basic "rock" foundation of their music, the larger commercial success for Queen worldwide doesn't really come as a surprise for me.
On the other hand there's Metallica, a band that evolved from a raw thrash ensemble in the early 80ies over a band that garnered some Mainstream success in the 90ies with their black album (I won't say "major" mainstream success, for even though Nothing Else Matters was a huge hit, they were not able to follow up that level of mainstream attention ever again, not even with other hit singles such as "The Memory Remains" or the St.Anger-sinlges), which served some tuned-down Metal compared to their older, more aggressive stuff, but who essentially always remained one of the leading acts in the Metal scene more so than in any "Mainstream".
Now while that doesn't say anything about the quality of the music, it most likely says something about the fan communities - I believe a die hard Metallica fan will hardly find the music of Queen attractive, and vice versa. I, as an avid fan of European Power Metal, love both bands - Metallica for their work they did for the foundations of "Heavy Metal", with their heavy duty riffings and also for their work in making Metal "socially accepted" in the 90ies with the success of their Black Album; and Queen for their brilliant work with choruses and great harmonies that paved the way for the entire genre of (Euro) Power Metal more than any other band did; without Queen's Bohemian Rhapsody, Innuendo, The Show Must Go On etc... I believe we would never have seen bands like Helloween, Blind Guardian (who actually also named one of their albums "A Night At The Opera") or Stratovarius, who created a genre of their own in the 80ies, which is heavily inspired by the works of Queen.
Now I can understand someone who likes Metallica, which is a band with a more socially critical (especially in their great lyrics, which I consider among the very best Metal has to offer), no-nonsense attitude that doesn't rely on "show" elements that much, but focuses on "kicking butt" with their music; whereas Queen have always been much more flamboyant and over-the-top, both in their music and also in their image; and naturally I'd say that those two very different styles would almost inevitably provoke conflicts. Personally, as said, I like both bands - but for my personal taste, Queen was much more important - both because they were more accessible to a greater group of people, not only because of their work on the movies Highlander and Flash Gordon, and because they laid the groundwork for many Power Metal bands of the 80ies - and also had the musically much more interesting and sophisticated material of the two. Metallica? Great band, but to me, not the best or most important Rock band of all time. But a very deserving second place.
The one advantage Metallica (might) have, though, is that they're still going strong - and while I highly doubt they'll ever regain the importance they had in the 80ies as one of the "founding fathers" of Thrash Metal, they still have a huge fan following today, and nowadays music stations, MTV etc... are all much more likely to play Rock and especially Metal again than they were in 90ies - so together with their fan following and more promotion on mass media, I believe it is a safe bet to say that Metallica will have more success in the years to come than Queen, even if they are doing new albums and tour now. Because generally, the Queen fan demographic is an older one and I believe that statistically, older people are less likely to purchase any CDs than the younger following Metallica has - so that will give Metallica an additional boost when it comes to CD sales and chart entries.
And nonetheless, no matter how successful Metallica will still be in the future (and even St. Anger was a very big success, and I personally hate that album because of its sound) - for me, Queen has still done more, or at least, the more important things throughout their career than the Mets. But who knows what is yet to come? At this point in time, for me the greater Rock band of the two remains Queen.