Sonar is a great composition tool - esp. w/ midi. FLS is not, tho on the forums there used to be plenty of works that were for more complex than I imagined possible w/ the tool. But still, FLS would not be my choice, tho it is wonderful for producing beats and is a lot of fun. Basically, any DAW is capable of making scores. Your question is akin to asking which Word processing program is best for writing. They will all work.
For film/video you'll need VSTs, esp. large orchestral synths. These are sample-based, usually, and there are many to choose from. Sonar Pro provides some good synths, including dimpro, which has some nice orchestral sampling, tho not as complete as East/West and other large libraries. Dimpro is more of a lot of stuff rather than having a gigabyte of triangles with every possible hit/stroke of the instrument. There are libraries for it, and a lot of SFZ libraries. For orchestra I'd get Kontakt (the sample engine) and add libraries as you need them. Lots of pertinent stuff for Kontakt since it is the most popular sample engine around. If you wait until Christmas you can pick it up cheap (probably). Libraries like Heavyocity (you can hear there stuff all over TV) will also probably go on sale. Kontakt, combined w/ SONAR and its synths (plus a few other nice free VSTs) ought to give you enough to get started and not feel too limited. And once you get everything working, you don't need to upgrade your computer and system until you want to. I still keep an XP computer for a couple of programs that don't work on higher OS's. In fact, "upgrading" is a pain in the neck and shouldn't be a concern until you have mastered your system and you absolutely need to. It may be simple - it may take weeks/months to get it working as well as your old one. These days, computers are fast enough so you don't need to upgrade every year or two, unless you just like the bleeding edge.
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