I'm thinking of getting a new backup camera to go with my HXR-NX5U. I shoot 1080p/24.The cameras I'm looking at only shoot 60i. What is the downside with mixing these two formats in the timeline? I'm shooting a documentary, so I don't know if the difference will matter that much. Or will it? I'll be editing using FCP 7.0

The downside is that, IMO, 1080i60 looks positively horrific when rendered as 1080p24, but that's just my opinion and is based on my experience with Adobe Premiere Pro CS4. Perhaps FCP has some magic Premiere doesn't in this regard, but I kind of doubt it.

Of course, if you have the NX5, you can just try it and see for yourself. The NX5 shoots both formats, after all!

If you want 1080p24, you should get a backup cam that shoots 1080p24. If you don't care, then fine, get a backup came that shoots 1080i60. But if you do that, set the NX5 to 1080i60 too and do everything as 1080i60!

My recommendation. :-)

Best,
Aaron

I totally agree:
The visual cadence of 24p source is drastically different from 60i source, and no matter which one you think looks best, if you intercut the two in a movie, it's going to be obvious.
Viewers may not know exactly what's going on, but they will be aware that something "doesn't look right".
You can fudge a on bit mixing 30p with 60i if you are careful, but if you are shooting 24p at all, you should really consider shooting the whole movie in 24p.

I knew I came to the right place.