Even though I love the small local 5K races with their lack of hassles, I'd have to say I'm more likely to run well in a large-field marathon. I've only done one of under 1000 and that was an unpleasant experience due to the lack of traffic control and fellow runners.

I mean, if you finish a full marathon and there are just a handful of runners and volunteers, etc. in the finish area, it sort of feels like you just ran the local
fireman's 5K. Not very festive at the finish considering what you've done.

That said, I'm speaking as someone who is always closer to the front than back. If I had to run in those super-thick packs of runners wearing Spiderman costumes or Statue of Liberty garb, I might seek out the Kiwanas marathon with a 400-runner limit.