Nikon's 400mm F2 or Tamron's 200mm F2?

I currently shoot with Nikon D7000 upgraded from Nikon D60. I have been shooting a lot of sports games, and a few have been at night. When shooting at night my pictures "suck." I shoot with a 300mm F5.6 & my question is for shooting sports at night should I invest in Tamron's 200mm F2 lens I say Tamron because I can't afford Nikon's F2 lens. I have also been thinking of buying the Nikon 400mm F2 for my day shots. I guess I'm really stuck on whether or not the Tamron 200mm is any good in very low light shooting.

Both are going to be good for low light shooting due to there f/ values. The quality of glass is going to be the question, i've always in my experiences had better luck using the main brands lenses. My recommendation is to go with the Nikon, it may be more expensive now, but that also means it will have better resale value later on should you need to resell. Either way upgrading your camera body tot he D7000 is a good move int he right direction as well, better quality ISO at higher levels will help in whatever you end up choosing.

I shoot lots of sports, and have never seen a Nikon 400mm f/2 on the sidelines. I guess most sportsphotographers aren't too comfortable with a fixed focal-length $6000 lens in the trenches. As for the Tamron 200mm f/2, I can't find any trace of that lens in Tamron's catalog or anywhere online.

For sports afield shot at night, a Nikon 70-200mm f/2.8 is the weapon of choice. Personally, I use a Sigma 70-200mm f/2.8 (budget) and find it gives me great results on my Nikon D300. For longer shots, I compromised and use a Sigma 135-400mm f/4.5-5.6.

Here's some night games, shot primarily with these two lenses.
http://www.sportspagemagazine.com/content/fb/hs-fb-min/gal-hs-fb/cif-football-sac-joaquin-division-ll-finals---gran.shtml?44004
http://www.sportspagemagazine.com/content/fb/c-fb/gal-c-fb/kraft-fight-hunger-bowl-nevada-20-v-boston-college.shtml?44781#4
http://www.sportspagemagazine.com/content/fb/mp-fb/gal-mp-fb/ufl-sacramento-24-vs-florida-20---part-1.shtml?42193#7