I think it is very likely its gonna be Tool and im very happy because of that. I am truly glad that members of both bands recognize the magnitude of the other band, and i think tour (which was offered to Opeth by Tool two times already!) would be like the final realization of artistic understanding and mutual respect.
Even though it was not the original subject of this thread, Tool/Opeth comparison became like the main topic, so ill tell my thoughts as well, i think its worth elaborating on that. Tool and Opeth are like my two favorite bands, and their music affected my life like no other. So, when im comparing im not trying to participate in some competition or something. I just wish to make a few observations as to which qualities i perceive in each of the band's music, without any better/worse implications.
I think its been fairly noted that no other band has such developed and refined "brainy" and intellectual component to its sound as Tool does. Intellectual snobbery of some fans aside, Tool's intellectualism is very defined and transparent, prominently in their art, but also in their attitude towards politics and music business. I think it gives them extra integrity, and for me they are an epitome of independent creativity, true "alternative" to everything i dislike in world and in music. There is this rational element to their music, which is evident in songwriting, in Adam's mellow delivery, and Danny's highly cultivated, "mathematical" drumming (you get the sense he just thinks what and how hes gonna play), but it is perhaps best fulfilled in Maynard's interpretation of his brilliant and literary lyrics. Even though theres a lot of mystification going on, as well as this recurring and annoying notion that his lyrics are "pseudo meaningful", i have to say that i have never found a singer with such poetic gift, such ability to use words in highly sophisticated and metaphorical (or even, if its needed, direct and to-the-point) way and make like a parallel layer of poetry flowing along with the music. And Maynard's lyrics are indeed intelligent, informed and often very challenging and complex; however, "intellectualism" never goes to the point of degeneration, it is always only one half of the whole picture, and emotional and intuitive half is always as important. So it is all very dynamic and very organic. Finally, Tool's music is the result of four men working together and contributing (hypothetically) equally, it is "splitting everything four ways", as Adam once put it.
Opeth is something else. Not that theres no sense of intelligence or understanding, but i think it is emotional side that is prevailing. One can definitely sense how awareness of the form progresses from album to album, but it is still somehow unconscious and spontaneous. I think Mikael has awareness of the form, but he is still very much a "volcanic talent". He also does everything with incredible ease and productivity (unlike Tool who seem to struggle a lot more through the process of songwriting). In case of Opeth, music is much more important than lyrics, as lyrics seem to merely complement the atmosphere created by music, and final result is very evocative and dare i say sensuous. You either respond to the music or you dont. It is deeply connected with some archaic and wonderfully irrational core, almost as in shamanism. On the other hand, many Tool's song will be fuller understood and much better enjoyed only when some kind of key or concept is deciphered (hence many find Tool "boring", "too slow" and so). Lastly, without taking away anything from every member's amazing input, lyrics and music of Opeth are result of a single man's creative vision. It is one crazy unrestrained Mo-Fo coming up with all the ideas and that is something that should be noted, even if it eludes explanation. Mike may not be an educated bookworm like Maynard, and he may not be keen on being politically outspoken or defiantly "alternative" in his taste and attitude (unlike Mike, Tool guys loath being labeled "metal"), but to me no one can beat him musically, not even such amazing ensemble of 4 that is Tool.