Hi,
I have not used either of these lenses (but have used other lenses in that range), so I will not comment on these specific lenses. I recomment you visit
www.photozone.de or
www.slrgear.com for lenses reviews.
Things to consider:
1-lenses such as these will always create some level of distorsion, chromatic aberrations, and will probably be of good, but not great, resolution. They compare well with kit lenses however.
2-these lenses are relatively "slow" (not letting much light in) at the 300 mm end. That means image stabilization (called VR by Nikon) is a big plus, and a tripod even more so, when light levels are low.
3-I do not know much about the Nikon but the Sigma and Tamron are well regarded, as long as you nderstand thir limitations. The Tamron is generally said to be sharper but has more aberrations too.
I do not know of any other "consumer" options in the Nikon lineup, but I'm not really familiar with the Nikon lenses (I shoot Pentax). One other solution, which might be more expensive however, would be to get a prime (non-zoom) lens in the 200-300 mm range. You'd get a better lens but would loose the option to zoom in or out. Primes in that range might be expensive though, because they are often quite good.