The advantages of most grands are: greater dynamic range (loud/soft), open structure allowing the sound to escape freely (which provides more control and expression), richer sound, faster repetition, smoother action (use of gravity instead of springs), a "true" soft pedal (see section II.24), clearer sound (easier to tune accurately) and more impressive appearance. An exception is the class of "baby" grands (less than about 5'-2") whose sound output is usually unsatisfactory and should be considered mainly as decorative furniture. A few companies (Yamaha, Kawai) are beginning to produce baby grands with acceptable sound, so for these very new pianos, don't write them off without testing them. Larger grands can be classified into two main classes, the "student grands" (those below about 6 to 7 ft), and the concert grands. The concert grands provide more dynamic range, better sound quality, and more tonal control.
As an example of this "quality versus size" issue, consider the Steinway pianos. The baby model, model S (5'-2"), is essentially a decorative furniture and very few produce sufficient quality sound to be considered playable and are inferior to many uprights. The next larger size group consists of models M, O, and L (5'-7" to 5'-11"). These models are quite similar and are excellent student pianos. However, advanced pianists would not consider them to be true grands because of poorer sustain, too much percussive sound, and notes with too much harmonic content. The next model, A (6'-2"), is borderline, and B(6'-10"), C(7'-5"), and D(9') are true grands. One problem with evaluating Steinways is that the quality within each model is extremely variable; however, on average, there is a significant improvement in sound quality and output with each increase in size.
One of the biggest advantages of grand pianos is the use of gravity as the return force of the hammer. In uprights the restoring force for the hammer is supplied by springs. Gravity is always constant and uniform across the entire keyboard whereas non-uniformities in the springs and friction can create non-uniformities in the feel of the keys of an upright. Uniformity of feel is one of the most important properties of well-regulated, quality pianos. Many students are intimidated by the appearance of huge grands at recitals and competitions, but these grands are actually easier to play than uprights. One fear that these students have concerning these grands is that their actions may be heavier. However, touch weight is something that is adjusted by the technician regulating the piano and can be adjusted to any number regardless of whether the piano is an upright or a grand. Advanced students will of course find it easier to play demanding pieces on grands than uprights, mainly because of the faster action and uniformity. Consequently, grands can save you a lot of time when you try to acquire advanced skills. The main reason for this is that it is easy to develop bad habits when struggling with difficult material on uprights. Challenging material is even more difficult on electronic pianos (and impossible on models without proper touch weight) because they do not have the robustness and response to touch that are required at high speeds.
Some people with small rooms agonize over whether a large grand would be too loud in such a space. Loudness is usually not the most important issue, and you always have the option of closing the lid to different degrees. The maximum loudness of the medium and large grands is not that different, and you can play softer with the larger grands. It is the multiple sound reflections that are most bothersome. Multiple reflections can be easily eliminated by a carpet on the floor and sound-insulation on one or two walls. Thus if the piano physically fits into a room with no obvious difficulties, then it may be acceptable from the sound point of view.