I vote flash, feel free to send me any gaming footage you would like to be put into a montage though, this goes to anyone, I have editing skills and would love to put them to the test.
Considering i have no clue what an XNA Video is, and flash is, as lax put it, expensive, i vote montage...wish i had a capture card so i could help you out. i always thought it'd be kinda cool to be in/help with a montage/short.
Ah...well in that case i still vote for montages/vids/etc. Heres my view of the plan...
1. You get capture card 2. Gamerland community plays Halo, or whatever else 3.... 4. Profit/Become internet celebrities (Beefy song plays in back of head)