Not really. Ironically it's less work to initially develop layouts on a windows platform instead of mac if the solution will be used on both systems. That's because text of the same size and font takes a bit more room horizontally and vertically on windows systems. Thus, a layout developed on a mac may contain text that is clipped or wrapped incorrectly when viewed on a Windows machine while the switch in the other direction usually does not. (Admittedly this is a very thin reason to prefer Windows over Mac, but the differences are pretty small in most areas.)
On the other hand, if you like to use New Window to pop up "Pseudo dialog boxes" in place of Show Custom Dialog, the process works much nicer in Mac than on Windows (Where the parent window can resize when new window executes in certain circumstances) so take your pick...
+1 to what Phi says.
Additionally, if your primary target market is for a Windows environment, then there is no compelling reason to switch. However, it is good practice to be able to check how your solution looks on the other platform, and also on the mobile devices that are natively supported (iPhone/iPod touch, iPad with FileMaker GO). This rationale was my very "thin reason" to get all the above-mentioned products and to do my development on a MacBook Pro running Windows 7 within VMware Fusion! :) ... worked for me ;)
At the risk of hi-jacking this thread, I have just yesterday run into the "parent window resize on new window execute" issue on Windows; Mac is fine. Any thoughts on how to remedy this?
I develop on a Windows 7 machine. A Mac client called me recently, after she bought a new iMac. She also upgraded Filemaker from v9 to v11. She said the fonts were blurry and some barely readable. They looked fine in Windows. So I put FM on my new Macbook Pro and the fonts looked fuzzy on it as well. I tried FM9 as well but the font problem persisted. I have since un-bolded anything that was bold. I only use Arial. I took screen shots of the Mac screen and the Windows screen. I looked at them on both machines and have come to the conclusion that the problem is the way fonts are rendered on new Mac hardware. A friend opened my database on his older Macbook and sent me a screenshot and the fonts look fine. I tried Lion and Snow Leopard - same result.
I think if I did my layout on Mac, the text may look thin on Windows machines.
The only advantage of Mac over Windows that I've found is the ability to customize the toolbar with smaller icons. Not so in Windows.
With regards to the "windows window resize issue" take a look at the scripts used in my Known Bugs List database. These use get functions to adjust the size of the parent window back out to the largest possible dimensions without actually maximizing the windows. Then, when a script closes the Pseudo Dialog, it also resets Filemaker back to maximized windows to return things to their original state.