For general pictorial use, you're a lot better off pulling a faster film than pushing a slower one. The pulled ISO 400 film will yield much better shadow detail than an ISO 100 film pushed a stop.(posted 8384 days ago)Basically, pushing does not increase film speed; it only adds density to the negative in the middle and high values, and does next to nothing to the lower values. Thus, it will allow printing an underexposed picture with reasonably good results. However, the shadow values will lack detail. Film which has been overexposed and underdeveloped will yield additional shadow detail at the expense of local contrast.
Which is better? If you had to choose one or the other for everything, then overexposing and pulling will yield more consistently good results. With that said, however, in flat lighting, where there are no deep values and the range of brightness does not exceed 4 stops, you may find the pushed film will produce a more satisfying result because it will separate the mid-range values better and the increased contrast will then be benefitial. Conversely, a pulled film under the same conditions will look flat and will require higher contrast paper when printing. In bright, contrasty conditions, there is no camparison--the pushed film will be very hard to deal with (you almost have to expose for the highlights and completely sacrifice detail in the shadows), while the pulled film will show good detail in highlights and shadow.
Another thing, the difference in sharpness and grain will be lessened by pulling a fast film and pushing a slow film. The ISO 100 film will still yield greater sharpness and finer grain, but not as pronounced as when both are exposed and developed normally.