I'm working on a large program which has an abstracted GUI API. It is very GUI based, many dialogs and a few nasty features which rely heavily on the message flow of the GUI (correct sequences of focus/mouse/active handling etc.) - not easy to port

I now want to port it from the currently used FOX Toolkit to native Cocoa/MFC.

I give myself a timeframe until the end of the year but my main work will be to continue development work with the existing toolkit, but there is no planned release for end customers before both tasks are done.

My question is how should i spend my time?

1. Stop working on the main program and do a 90% port (about 3 month) of the GUI first
2. Splitting everything into smaller sessions of one month each.
3. Assigning Monday/Tuesday to the GUI project and the rest of the week for the app.
4. Finishing the App first, then port.

I think there are three arguments which i need to balance.

1. Motivation, i want to see something going on on both projects
2. Brain Input Overflow, both tasks require a lot of detail information in my brain and sometimes enough is just enough.
3. I guess the porting is intervowen so porting would also require a lot of code changes in the existing code and the new code that will be written in the meantime.