Unfortunately, the sort of subsidized purchase noted above is not available in the US. (Several reasons for this but that's outside the scope of this thread.) On the broader question of whether to tether (at least in the US) there are several considerations.
() I haven't seen a direct comparison between the 3G speed of an iPad versus that of the iPhone. But a test published today on Gizmodo confirms that the download speeds of the iPhone 4S is little better than the iPhone 4. And each is very slow compared to the 4G speeds available on several Android phones. In other words, Apple's claim of much faster 3G performance in the iPhone 4S seems to be marketing fluff.
This seems to cut both ways. On one hand, you're nearly as well off tethering to an iPhone 4 as a 4S. On the other, both are relatively slow. But, as noted, I haven't seen how either compares to the performance of 3G built into the iPad. I suspect, however, that it may be a bit faster. (Most native 3G devices are faster than devices tethered to 3G devices, especially if the tethering is wireless.
() If you want blazing download speeds on the iPad, the best way to achieve it is with a wifi iPad working with a mobile hotspot that is 4G enabled (and in a 4G service area.) That approach saves (in the US) $130 on the price of the iPad but it also may commit you to a two year data plan that cannot be turned on and off.
() From a cost standpoint, if you already have a smartphone (iPhone or another brand) for which you have a data plan that you don't max out, tethering is a good alternative. You can save $130 on the purchase of the iPad and pay only the tethering monthly charge $20 on Verizon.
There are, of course, ways to tether a smartphone without paying the monthly tethering charge but doing so violates your TOS (contract) with your carrier, may require jailbreaking (or in the Android world rooting) your phone, and should be undertaken only if you understand the risks, including cancellation of your service or an imposition of a monthly charge you didn't sign up for. (Check your contract. Your carrier can do this unilaterally.)
() If you have an "unlimited" (or high ceiling) data plan on your phone, you may not have to worry about exceeding your monthly data allowance if you tether. But some carriers (AT&T and Verizon, for example) will "throttle" (slow down) your internet download speed if you consume too much data. That's likely only if you're watching a lot of streaming video content on your iPad, but it's something to keep in mind.
() If you don't want to worry about any of this, paying the additional $130 for your iPad and getting a 3G data plan for it is probably your best alternative. The advantage is that it's (currently) cheaper than a data plan tied, for example, to a mobile hotspot and you can turn it off and back on when you need it. But if you use 3G even once in a thirty day period, you'll pay the monthly charge so keep that in mind.