Moved to it's own thread here:
Discuss:

It depends on if you want something easier to plan or more difficult.

The first idea seems inherently easier to create since most of the PCs will be driven into one place and then fight, come back and repeat. Simple go and do type quests.

The second idea would need a bit more planning as you would need to flesh out all the why's who to talk to, many NPCs for information gathering and the like. Along with good ways to keep the players moving along.

Now if you can pull it off the second one is going to make for much better RP and most likely more fun. The first idea would be good if you have never DMed before. And just as a sidenote, you could even start out with idea 1 as the players are sent out to defend different areas, until finally the second idea happens indicating that largely the military is failing and unable to protect the country/nation/city state the players are trying to defend. This may help bring in a good story arc, and transition from being able to get comfortable with your players into a more RP centered game.

You could always ask the players if they have a preference.

yea that was going to be my sort of fall back plan. iwas just seeing what others thought also

I would suggest a defense/seige scenario to start with.

It has lots of advantages:
1. keeps all the PCs in one place to start with.
2. has threads to follow up
3. gets them into a fight straight away and tests our their builds, clarifies rules etc
4. sorts out the players who are committed to the game
5. easy to plan - have some scenary, objects and waves of attacks

I would recommend that you subscribe to Dungeon and Dragon Insider and read some of the material that is presented in the magazines. This is what Im doing. Like the advertisment says." Less prep more time playing." What I'am doing in my games is taking a few similar adventures or short mini games and running them in a consecutive order. There are plenty to pick from and you can add your own twist to the development of the game. I know its not very original but with players of that size you might need a little help with organizing the game. Might I also mention that there are maps and pre ready monster encounters. YOu also would have acess to the encounter builder and the compendium which you could just type any thing your looking for in all the published books and within seconds have all the source material at your finger tips.