Maybe consider Xojo, which is like a cross-platform Visual Basic. I know, that means you're programming in Basic, but it's the fastest tool I've found & the compiled binaries are relatively small (Mac version of my current app is 11 MB uncompressed). It supports Mac / Windows / Linux, but it also compiles to Raspberry Pi, iOS, Web, and Android support is in the works. Pricing is cheaper than Delphi or Qt. But if you need to collaborate, there aren't as many Xojo / REALbasic developers out there, as something like Qt:
http://www.xojo.com/
For other Windows/Mac stuff I'm working on, I use C++ for the cross-platform code, and Win32 API / Obj-C for the platform-specific interface layers.