Two issues I'd comment on.
First, you don't mention what platform you're targeting. Although recent Windows versions (Win2000, WinXP, Vista and Win7) support both Multibyte and Unicode versions of system calls using strings, the Unicode versions are faster (the multibyte versions are wrappers that convert to Unicode, call the Unicode version, then convert any returned strings back to mutlibyte). So if you're making a lot of these types of calls the Unicode will be faster.
Just because you're not planning on explicitly supporting additional languages, you should still consider supporting Unicode if your application saves and displays text entered by the users. Just because your application is unilingual, it doesn't follow that all it's users will be unilingual too. They may be perfectly happy to use your English language GUI, but might want to enter names, comments or other text in their own language and have them displayed properly.
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