Hi, I m actually looking for the superzoom recently also, just for the trip. Has been posting another thread under Canon.
From what I know, nikon vr2 will be the leading brand for this category. The rest will have their own strengths & weaknesses.
As such, i supposed if you are not comparing to the L lenses, it will not be big difference to get any of these lenses.
Thamron 18-270 seems yummy to me. But together with sigma, both of the lenses has this softness issue at the tele end say 250/270mm.
So, maybe really no point to get them compare to nikon/canon 18-200, if budget allowed.
Btw, I m using canon.