Hi! You don't specifically say which Nikon lens you are considering. I have an older "G" lens and it is pathetic!! Extremely soft after 200mm and ultra slow focusing.
The "D" lens is not a great deal better from all accounts... read Rockwell's review
HERE. Bear in mind that like most reviews, it's just one man's opinion! He actually refers to the Tamron too in the review.
The newer
"VR" model might be the one to go for if you can afford it. My experience of VR is very good. I have an 18-200mm and think it's a cracking all round lens. These are what I would consider to be "serious amateur" lenses.
As for Sigma, I have 3 lenses from their "EX" range and love them all. Even the ridiculously long-range 50-500mm is a smooth operator. They represent excellent value for money compared to the Nikkors. Can't say I'm too keen on the matt finish though.
Regards, Tom.