Ibanez really does make a good 7 string. I think if you found a used UV777BK you would be very happy with it. The other companies you mentioned also make great guitars, but Ibanez guitars have always seemed to be the best for me.
i have 2 ibanez 7s.. i've never played the petrucci 7 but i have played the 6 and they are very nice.. as far as esp ltd, schecters, bc rich, jackson, and anyone else who has made them.. i haven't found any to be as good as EB or Ibanez. I've never played a carvin, but i believe many think they are pretty nice instruments
I've tried Petrucci's Musicman but I don't think that this guitar is worth that much money. I own 4 Ibanez 7 strings (Universe 777PBK, 2077 XL, custom 1527 RB with Blaze Custom&Blaze neck, custom 1527 BP with ToneZone7&AirNorton7) so I can compare. Definitelly go for Universe (if you want to spend a little bit more ), or 1527 RB, this is fantastic guitar and it doesn't cost so much.