I would probably used iWork if it was available for Windows since it's a lot cheaper and it seems like the things you can do with iWork look more professional than Office (maybe that's just because I've seen so many bad PowerPoints).
Presentations done with Keynote instead of PowerPoint usually look more professional, even when they're done by a child in grade school... Not even joking.
The matter of which is more professional looking lies on the one that uses the program, as far I know both keynote and powerpoint have features that allows you to make very good presentations. But to which I am most likely or prefer to use, I would probably say, iWork, because it consumes less resources and as James said it costs much cheaper.
Apple charges a lot less for a lot of their software, which I guess the only other thing is Mac OS X Leopard. Leopard is way cheaper than Vista and comes in 1 version, which Microsoft hasn't figured out that that's what they need to do so they don't confuse the consumer!
Not to get off topic, but if you look at the actual development costs of Apple's software vs how much revenue they make from software, they come pretty close to breaking even. They don't make nearly enough profit to sustain the company. Essentially 90% of Apple's profit is made from hardware.
Microsoft on the other hand is a software company. They don't make hardware with which to inflate prices. This is why their software is much more expensive.
Don't be fooled thinking Apple's software is cheaper. You're just paying for it in a different form. ;)
Well if you're looking for cheep, I know that Open Office is out for mac OS X, and in my opinion Open Office is really good at a lot of things. Then again I also believe that you have to have some basic skills in order to use it.
One of the common pitfalls is the open format that isn't compatible with iWorks or MS Office and have led schools and public administration to abandon the use of OO entirely, which is really sad since with a click of a button you could get any format you wish for from OO and it is one of the areas that I think Open Office is great at.
iWork is makes more professional looking documents and keynote kills powerpoint for me. Overall i think iWork is better. And its cheaper. and its more stable when using numbers. although none of the products do good when doing serious number crunching. They both fail