UTF8 vs. Unicode (UTF16) in code

We've got an English-language only product which makes use of
single-byte character strings throughout the code.  For our next
release, we'd like to internationalize it (Unicode) & be able to store
data in UTF8 format (a requirement for data exchange).

We're considering between using UTF8 within the code vs. changing our
code to use wide characters. I'm wondering what experiences others have
had that can help with our decision.  I'm thinking that using UTF8
internally may mean less rewriting initially, but we'd have to check
carefully for code that make assumptions about character boundaries.
Because of this, I think that it'd be more complicated for developers to
have to work with UTF8 in code.  Unicode (UTF16) internally would be
easier to manage since most characters will essentially be fixed width,
but there'd be alot of code to rewrite.  Also, I've heard of problems
with the wide character type (wchar_t) having different definitions
depending on platform (we're running on NT & Sun Solaris).  Many of our
product APIs would also be affected.

Can others offer their insights, suggestions?

Thanks,
-allan