As a beginning user of Keynote (check HERE for a description), and a longtime user of PowerPoint, I've opened a number of my PPT files in Keynote - the program is not as powerful w/ the fonts and formatting, so a number did not open identically but I am still impressed. Keynote will save in a variety of file formats including those you listed - cost is $10. Good luck in your selection - have not checked into other apps that may open PPT files -
There are better apps than KEYNOTE, imo. You could look at Documents to Go or another similar app. Keynote is great, but for the same price if not just a little more, you can have a better service.