Need help deciding Nikon 300mm lenses?

On Ebay, the Nikon 70-300mm Vr is around $500.
and there's the Tamron 70-300 VC lense for $400.

Obviously quality wise, the Nikon is better. But the Tamron is much cheaper and there's a lot of positive comments about that lens. Anything that I should be aware about or consider between the 2 lenses? I'm thinking about getting the Tamron but any advice is better than no advice!

Added (1). Also can this lens be used on a DX AND FX camera? I own a Nikon D90 but if I want to upgrade to a full frame in the future, will this lens be compatible with it too?

In terms of price, 100 dollars isn't much cheaper for a third party lens. More expensive lenses such as the 70-200 2.8's are sold way cheaper by third party manufacturers. The reason why the 70-300's are so close in price is because they can sell them that way. Don't be fooled, be smart and grab the Nikon, you won't be sorry!

From Tamron's own website.

Tamron Di lenses are performance optimized for digital SLR cameras and usable with DSLRs featuring sensors commonly referred to as APS-C, measuring approximately 24mm x 16mm, as well as larger full frame (36mm x 24mm) sensors and 35mm film SLRs.

I've also read very good things about that new tamron. If you don't need the weather resistance of the nikon it's certainly worth considering. If I was a fair weather photographer in need of a 70-300 Id buy the tamron

First and foremost, you did not elaborate on whether the lenses were new or used, or what conditions they were in. If it is a used lens (which should not be for the prices you quoted), then lens condition is going to be more important than brand.

And I would be very careful buying lenses from eBay.

Otherwise, I agree with the above posts that $100 is not a lot more to pay for a Nikon. And if both are new, the resale difference means you may more than get your $100 back if you ever sell the Nikon.

While it depends on the particular lens, Nikon lenses in general seem to hold their value - especially the pro lenses - a lot better than the 3rd party lenses.

At any rate, I do not generally like Tamron as I believe they are inferior. However, I do realize that from time to time they do make a decent lens (and one of the vintage Nikon lenses I used to own was actually made by Tamron on contract). But otherwise, I would never buy a Tamron lens especially if it was only 20% less expensive than a Nikon.