What is the difference you might ask? Since pokies are really casino slot games in basic terms, the name shouldn't affect the difference at all. But indeed there is a big difference here since games played on your mac and games played on the iPhone are both in need of clarification.
Mac is usually what you home computer is called, even though your iPhone is made by Mac it's still an iPhone or iPod. No difference there. Although your Mac can be a laptop as well. Many pokies can be played on a Mac with either flash or downloaded. The memory of a Mac can support the amount of space that is required for downloads. Usually 100- 300 MB per game and software included to run it. The screen is usually bigger and it can't fit into your pocket so easily unless you happen to have really big pockets.
The iPhone however will support the flash-based games very easily with direct support from the internet but cannot support so much in the memory to store these kinds of games. Even with 512 MB in the iPhone 4. (the new iPhone 5 reportedly has 1 GB of RAM) but forget it, why would you use so much memory for an iPhone anyway?
And besides the mac vs iPhone debate, you can easily have the same kinds of freedom of taking iPhone anywhere that it's easy to access. This is why Mac will typically be the back-up for any Apple computer being used. You can't write a report or make a spread sheet on a iPhone so easily as you can't use a Mac to talk to someone in private.