As to which brand to choose, you are probably already leaning toward one. There may be a deep technical reason why you would choose one over the other, or you may have a preference based upon marketing or a recommendation. Both companies' products are reliable and they split the market almost down the middle. So if you don't have a specific technical reason for choosing one, either one should be fine.
As to what speed to select; the price for the latest, fastest CPU is always at the top of the price curve. As you move back to slower CPUs the price drops dramatically. The oldest nearly-out-of-production CPUs are downright cheap; but not that much cheaper than the middle of the pack. Therefore the best bang for your buck is usually in the middle of the speed range. If the currently available CPUs ranged from 900 MHz, 1.2 GHz, 2.4 GHz through 2.8 GHz and 3.2 GHz, the 3.2 GHZ would be too expensive. The 900 MHz would be cheap, but the 1.2 GHz would be only a few dollars more and the 2.4 GHz only a few dollars more than that. So the best deal is usually the CPU that is about two speeds slower than the fastest currently available.
Another aspect of choosing a CPU is whether you have some specific CPU requirements to run the software you want to use. You have to meet or exceed the speed requirements of each of your software programs. This isn't hard to do because the CPUs today far outstrip the requirements of nearly all software. You should anticipate the demands of future programs you might want to use, but it is, at best, difficult to predict.