You don't need to own a grand, an upright, and a vintage electric to work out which one you like. You just need to play the same thing through each and listen. The sound you choose shifts the mood of a piece more than most beginners expect, so it's worth knowing what each one is for.
Here's a quick tour of the main keyboard sounds and where they tend to fit.
Grand piano
The default, for good reason. Bright, full, and expressive across the whole range. If you want something to sound like a real piano, or you're playing classical, ballads, or anything where the piano is the star, start here.
Upright piano
Warmer and closer than a grand, with a slightly boxier character. Uprights suit quieter, singer-songwriter material, the kind of thing that sounds like it was played in someone's living room. If a grand feels too grand, an upright usually feels more honest.
Grand or upright: what actually differs
Plenty of people arrive at this question about the real instruments rather than the sounds, so here is the short answer. It is not the number of keys. A full-size upright and a concert grand both have 88.
The difference is geometry. A grand lays its strings and soundboard out horizontally, which lets the bass strings run much longer and gives the soundboard far more area to move air. A concert grand runs to roughly 2.7 metres for exactly that reason. An upright stands the same assembly on its end against a wall, so the strings are shorter and the box is smaller. Longer strings and a bigger soundboard are what people are hearing when they call a grand fuller, more resonant, and more capable of going from very quiet to very loud.
The action differs too, and it matters more than the tone for anyone playing fast. In a grand the hammers sit horizontally and fall back under their own weight, so repeated notes re-set quickly and quiet playing is easier to control. An upright's hammers move sideways and need springs to return, which makes rapid repetition a little less willing.
Then there is the ordinary stuff: an upright fits against a wall and costs less, a grand needs floor space around it and costs considerably more. Worth saying plainly, though, that a well-maintained upright in a good room beats a neglected grand in a bad one. Condition and room acoustics decide more of what you actually hear than the shape of the case does.
Electric piano
The mellow, slightly bell-like sound you know from soul, jazz, and a lot of lo-fi. An electric piano forgives a simple part. Play three slow chords on one and it already sounds like you meant it.