If I'll be slowing footage down, what will look better? 30p or 60i? Specifically, I'll be filming archery--the arrows in flight. Wouldn't you think that since an arrow is so vertically thin that 60i would loose the arrow in some of the fields?

I think it would be easier for your NLE to do slow motion with 60i, since it would have more frames to work with and will be able to blend it better, however I'm not sure that's the case when it converts it to DV/29.95FPS

Technically, both 30p and 60i have the same number of frames per second, thirty. However, since 60i contains two discrete fields in one frame, there is twice as much motion information recorded as opposed to 30p. So yes, it is likely you could get superior slow motion with 60i.

As for "losing" the arrow when recording in 60i, that depends on how thin it is and how wide you are shooting. I rather doubt it myself, but if possible it would probably be a good idea if you could shoot some tests before hand.