OK, first off the 5D mkii is a bad ass camera no doubt. However, the D300 will kick its butt in low light. Second, DO NOT JUDGE THE 5DMKii HD VIDEO OFF OF REVERIE. That video was shot with a ton of L glass and primes. Do a search and you will see what lenses he used. Some are no longer produced. Third, I am not sure how much you have invested in Nikon gear but it might not be fully worth it to switch depending on how much is invested.

Ask yourself this, how often will you be using the HD to its potential? Will you be shooting any sports with the 5Dmkii? What are you mainly going to be shooting?

If you really want to go canon then the 5Dmkii is a great camera. The 24-105 is a great lens as well and offers a better range than the 17-40. Really comes down to your budget and if you can afford the glass to go with the 5D.