If you are looking at a 48" tank, you can look at 75 or 90.. I think the height is the main difference. It depends on what you are thinking of putting in it to help you decide. If you want fully planted then the deeper tank will need stronger/more lighting, ie more cost. Should you decide to go SW some day the 75 is cheaper to convert too.
Maybe none of this matters but they are things to consider (especially if you are very new to fish keeping and reading this thread). Having said that, the larger the tank, the more options you have for the fish you might like to keep, although unless one goes from 48" to 72 inches, (4' to 6') it restricts what you can keep long term.
I would say that the bigger tanks are easier to care for, but exponentially more costly. Bigger filters, more lights, more gravel, more plants, rocks.... etc. As for care, things happen slower in more water. Consider a drop of food coloring in a glass of water, vs the ten gallon tank. It diffuses more, less effect, slower reactions. This is why smaller tanks fail so easily. They get overstocked easily and too much ammonia builds up leading to loss of fish.
Looking forward to hearing what you choose to get!!