Unfortunately, this is a terribly complicated issue. And it gets even more confused and complicated by the fact that different carriers, different phones, and different countries make a huge difference in the conclusions one draws. I'll just add a couple of points that relate only to the US.
() Despite claims that the iPhone 4S performs significantly better than the iPhone 4 in terms of internet download speeds, that does not appear to be the case according to tests conducted by Gizmodo. So if you're hoping that the 4S will provide a much better alternative for tethering, that's probably not the case. The good news, of course, is that if you have an iPhone 4, you probably will get nearly as good performance from that device.
() The iPhone 3G performance is much, much slower than 4G service now available on a number of Android phones. (Not trying to start a war here. Just pointing it out.) If you want to (and can) benefit from 4G connectivity, a mobile hotspot or 4G phone is a better alternative than an iPhone in terms of performance. (My Verizon 4G/LTE mobile hotspot achieves speeds from 6 to 10 times that of 3G connectivity.) But keep in mind that I have a two year contract with a minimum charge of $50 per month (for five gigabytes) with that device. I can, however, use it simultaneously with five devices, including my smartphone that would otherwise be limited to 3G connectivity.
(Note: I happen to live in an area that has very good 4G/LTE coverage. The download speeds I get from my mobile hotspot approach those of a cable modem. Even if you don't have such coverage currently, it's likely it will be available to you soon. Carriers are rolling out such coverage rapidly. And apart from the inherently faster speeds, the 4G networks are much less congested than the 3G networks.)
() I have not seen any comparisons in download speeds between an iPad 3G and an iPad tethered to an iPhone. It's likely, however, that the former is somewhat faster. The usual difference in performance ranges from about 20% to 40%. You may or may not notice this difference depending on what you're doing. And as I said, I haven't seen any reliable comparisons between the iPad 3G and the performance of tethered iPad wifi devices.
() As others have pointed out, most users can save money by tethering their iPad to a smartphone and avoid paying for separate data plans. In the US there is the initial price difference ($130) plus the difference in the tethering fee ($20) and the monthly iPad data plan ($30 for 2 gigs on Verizon.) I'm not certain, however, whether the tethering charge can be turned on and off as the iPad data plan can be. If not, the savings may be less unless you use 3G at least once every 30 days.
() Keep in mind that your data download use may go up very significantly when you add an iPad. It's much more pleasant to watch a streaming movie on an iPad than on a smartphone, for example. Thus, you may exceed the monthly ceiling by sharing the devices. And even if you don't, some carriers (AT&T and Verizon, for example) may "throttle" your downloads once you reach an undefined level of use. The throttling will affect both your phone and your iPad if it occurs. This is likely not a major problem for most users, especially if your data use on your smartphone declines as you use an iPad more frequently. But it's something to consider if you're already bumping up toward the ceiling of your current data plan.