Hi Adrian,
I have the Sigma lens which I have used with my D7100 and my older D5100. I can't comment on the Tamron as I've not tried it but I think the Sigma is a good lens for the price - it can be had for $900 from Teds from example.
IMHO...
Assuming I'm using it right and don't have a bad sample, the main issue for me is that past 300mm image quality gets quite soft - it would be awesome if it were sharp all the way up to 500mm but for a $900 lens that's may not be a realistic expectation. It's great around the 150-200mm range.
In my opinion whether or not you're shooting stationary or fast moving subjects makes a difference with this lens too. The autofocus does tend to hunt a bit (it's moving quite a big bit of glass) but a quick turn of the manual focus ring to get it close helps a lot. If you're shooting in low light, the relatively slow aperture range on this lens might be an issue too.
The Sigma is a good budget or beginner lens, however my advice is to consider whether or not you can get a faster lens like the excellent Nikon 70-200 f2.8 instead - albeit at around 2.5x the price. I've never heard anyone who owns one say anything negative about them, and if the extra reach past 200mm is important teleconverters might be an option without losing too much in the way of max aperture.
On the camera body, the D7100 can be had at Aussie retail for under $1300, although the new D7200 just came out. Because the D7100 has a metal frame it's a bit sturdier when it comes to having a big lens attached than the cheaper consumer models - I've not been worried about breaking mine if I accidentally pick up the body+lens by the body like I did with the D5100 (plastic chassis). However with a big lens the rule is always to pick it up by the lens, not the camera body!! With the higher end models you get the benefits of higher resolution, more autofocus points, higher frame rates etc. that will be valid on any lens you put on the body.
Hope this helps, and happy shooting!!
Terence.
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