G9 has other advantages too
Yes, the S5IS has a more powerful zoom, but overall the Canon G9 is a more versatile camera, because of the following features:
1. Smaller size. The G9 will fit into a larger pocket, making it easier to carry around all the time.
2. Underwater case. This is optional, and it costs extra, but if you ever want to take the G9 snorkeling, surfing, diving to 130 feet, or evn just dunking it into your fish-tank for a closeup of your gold-fish, it's important that a waterproof case is available for the camera. There is no underwater case available for the S5IS, but there is one for the G9.
3. Add-on lenses are available for the G9, both a wide angle and a telephoto lens. This may compensate for the slight advantage of the S5IS's more powerful zoom. Actually, a 6x magnification power is pretty amazing by itself. Some 15 years ago, I paid big money for a 3x lens for my film SLR camera, and that lens was huge and heavy. Now, the Canon G6 offers a 6x zoom lens that is comparatively small, light and portable, and you can increase it's range with a 2X add-on, doubling your zoom to 12x. Yes, the S5IS can also use add-on lenses, ... but the point is that the G9 is truly versatile, and can even get the extended zoom range, if you really need more than the 6X provided.
4. More Megapixels on the G9. 12 Megapixels on the G9 versus only 8 on the S5IS. Granted, that may be overkill for most purposes. But, the greater number of pixels allows you the flexibility to digitally zoom in and/or crop the picture. Yes, you will lose some of the resolution, but you have plenty of resolution to spare! A few years ago, with only 3 or 4 MP cameras, I NEVER used the digital zoom, and advised others to disable it too. But, with a 12 MP camera, you can afford to use a 2X or even 4x digital zoom. Of course, this will lower you megapixel count by half, to only about 6 MP, but you will have an effective zoom range of up from 6x to 12X (on the Canon G9). Actually, the digital zoom can increase the zoom power 4-fold, ... but I would still advise to use it sparingly. The point is that a 12-MP camera gives yout this added flexibility, so you don't have to worry about the digital zoom. Use it when necessary.
5. Longer lasting Batteries. This is a toss-up. The Canon G9 uses Lithium ion batteries, which are better overall. They will allow you to take 240 shots (with the LCD screen on), as compared to only 170 shots on the Canon S5IS.
On the other hand, the Canon S5IS, even though it uses the less efficient NiMH rechargeable batteries, it does give you the option to use regular AA batteries instead, which can be a welcome feature if you find yourself at a place where there's no reliable power supply to recharge your batteries.
Overall, points 2 - 5 may be only minor issues. Maybe the biggest difference is in the size of the camera. If you want a camera that is always with you, it's a lot easier to carry a smaller and lighter camera such as the Canon G9. But, if zoom power is the most important factor for you, then go with the S5IS.