First ask yourself if the need it right this minute, if not hold off. But be warned they might not be released until the end of the year.
Apple looks set to release the new iPhone next month (maybe Sept 10) the iPad might follow soon after or the same time.
My bet is October is the earliest we can expect the Mac Pro, and iMac. Apple is likely to make a big deal with the Pro but they could be released at the same event.
If you can wait until later this year, just wait for the new iMac. If you need it now for school or work, well you dont really have a choice then.
As for which route you should go, kind of depends on what you need. I think the iMac would be a better, and probably cheaper, option, since you wont need to get a monitor or any other accessories.