1) most pros use Logic or DP these days, so you want to be familiar with one of them
2) the current Logic offering (the Logic Pro collection) at half the price ($499) of what it used to be, makes it a no-brainer.
Nuendo has many advantages over Logic. Here are a few:
1) It is cross-platform.
2) It has more features for scoring to picture than Logic
3) it has better virtual instrument handling, and unlike Logic, is able to pan a stereo file without using a plugin....!
4) Routing is much better, and PDC actually works properly (and has done for years).
Actually, unless you need the advanced features of Nuendo, Cubase will work just as well for you.
That said, if you are used to Logic, and don't care about the features that you've never used, then I'm sure Logic will be fine.
If you just need to score music for film, I would get both (Cubsae 4 and Logic Pro 8) instead of Nuendo 4, since this will offer you more options, you can always upgrade to Nuendo 4 from Cubase 4 whenever you think you need the Audio Post-Production tools N4 offers.
Having 2 DAWs IMHO is a good strategy, more tools, and if one DAW is stagnated in development/features, or gets buggy for any reason, you have the other DAW to keep you working happily, the only downside is the additional learning cuve you have to deal with to master two DAWs instead of one.