A Grand piano is considered better than an upright piano. Aesthetically I could understand why, but why does the orientation of the strings matter so much sound-wise?
It's all about the size, and therefore the length of the strings and the size of the vibrating surface of the wooden soundboard.
Even a baby grand at ~5 feet is longer than a typical upright is tall. A concert grand at 7-10 feet is much, much longer.
I can't do any better than what Wikipedia says, so I'm going to quote wholesale:
Really tall uprights do exist, but they're not very common. They are sometimes referred to as upright grands.
Of course, whether inharmonicity is good or bad is purely subjective. That classic "pub piano" sound, fits perfectly with some kinds of music. It's the sound of an upright, and probably couldn't be replicated on a concert grand.
Adding to the above, this is the reason why upright pianos went from having strings vertical to being overstrung. This means the strings ,particularly the lower ones, are diagonal across the soundboard.It makes them longer, with the above advantages, but still not as long as those in a grand. 36" on the bottom string of one of my uprights, compared with 42" on my baby grand.
The action makes a grand better.
In a grand, gravity helps the key return so you can play faster stuff. In an upright, the action goes sideways and has to be helped out by, umm, don't quote me on this, springs I think?
And the sound, too.
The sound is more of a subjective thing. But in almost all cases, a long enough grand will sound better due to the wonderful bass sound. Also they open more directly to the room so the string/soundboard sound gets to you more directly (plus the room reflections).
Uprights have to bounce the string/soundboard sound out the back against the dang painted drywall (your wall).
Also the pedals.
Grand pianos have a middle pedal called a Sostenuto pedal, which captures keys which are depressed and lets them ring while keys played after the pedal can play without being held on. Uprights on the other hand have a middle pedal that serves as a "practice pedal" which essentially mutes the whole instrument. Usually this middle pedal is missing or non-functional on an upright.
Grand pianos sound much better because they are better. :-)
Just kidding. Grand pianos are constructed a bit differently as has been noted, but the biggest thing is the sound is project up and out from the instrument and not towards the user with a lot of wood in one's face. An upright piano also has its action in front of the strings unlike a grand with the action underneath and striking the strings from behind. This also affects the sound, making an upright sound much different.
On an upright piano, the pedals work differently. The upright's una chorda pedal works by placing the hammers closer to the strings to make the piano play softer. This is a totally different mechanism where the hammers are moved over and strike two strings, rarely one string, on a grand piano. The tres chorda pedal on both works in a similar fashion with the string dampers are lifted off the strings to allow the strings to vibrate. The sostenuto pedal is a rare beast on an upright. They do exist but only on the really expensive ones. Many uprights that do have a middle pedal, are fitted with a practice pedal which puts a thin sheet of felt between the hammers and the strings. This creates a muted sound so the piano isn't as loud and intrusive, and allows for much later evening practice.
Overstringing does not increase the length of the strings. What does is create harmonics in the open part of the strings. Up until the early 20th century, even grand pianos were straight strung, with a mix of both straight strung and overstrung instruments around. Erard was still making straight strung concert grand pianos right up until the 1920s.
Here's a place that's worth visiting should you be able to. This is a collection of antique pianos that can be played on and heard. I have play on all these pianos listed on this website.
The main site is http://frederickcollection.org/