It would probably be best to stay with 1.9.4, it hasn't been out for long, so mods can actually update to it.
1.10 doesn't really bring anything new to the table and should probably be skipped, 1.11 will probably be out in 1-2 months at this rate anyway.
After fixing it by manually changing ${arch} to 64 in dev.json, I get the next error
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':downloadClient'.
> java.io.IOException: Server returned HTTP response code: 403 for URL: http://s3.amazonaws.com/Minecraft.Download/versions/1.7.10_pre4/1.7.10_pre4.jar
[/Code]
The correct URL should be "http://s3.amazonaws.com/Minecraft.Download/versions/1.7.10-pre4/1.7.10-pre4.jar".