Also - are there other reasons why you'd want to rewrite that code?
If it's well written in a language you're comfortable with supporting, I guess you can take on another expert dev in that language as you're growing.
Option 2 would certainly produce the fastest development setup.
I have realised that in trying not to cloud the decision by telling everyone what the tech was, I may have done you all a disservice by not giving enough information!
So here it is: The existing tech is Adobe Flex. Provides great RIA, but not good for iPhone/iPad. It makes up only the front end of the app, with everything else php based.
The new tech would be to re-write the front end in HTML/JS, leaving the backend php as is.
So another reason for rewriting the code is that we will likely want to dump flex at some point.