It should be a good show. If they were coming to Maine I would go. However, I would probably leave before Slipknot took the stage (getting home early would be a bonus).
I don't really like them anymore, but I saw Slipknot at the height of the nu-metal craze (99 I think?). They were terrible live. Joey played everything way too fast, the rest of the band couldn't stay in time, and Corey was literally always out of breath after the first song. They may have improved since then, but I think the whole mask/ costume/4-members-in-the-band-who-don't-contribute-anything-musically is pretty stupid.
I like Trivium, never seen them live but the clips I've seen they're OK (even though Heafy cares too much about his voice to do screaming parts anymore--he makes Corey do them).
Now, I'm a HUGE Coheed fan. I would be going just to see them really. I've never seen them, but in the clips I've seen they sound phenomenal. Not a metal band per se, but they have really obvious classic metal influences (Maiden) and they just write awesome music. For some reason they get lumped in with sh*t emo bands, but in my opinion they pick up where Rush left off.