The most common thing people get wrong when buying towels is not the weight. It is the size. A bath sheet looks like an upgrade on a bath towel (same cloth, more of it, a slightly higher price) and for a lot of bathrooms it is the wrong buy. It is worth understanding why before you order four of them.
What the words mean
There is no standard, so check the centimetres rather than trusting the name. As a rough guide across the industry: a hand towel is around 50 x 90 cm, a bath towel around 70 x 140 cm, and a bath sheet around 90 x 170 cm. Some ranges sell a 60 x 120 cm piece as a bath towel and a 70 x 140 cm as a bath sheet, which is why two shops can quote you the same names for quite different objects.
The arithmetic nobody does
A 70 x 140 cm bath towel is 0.98 square metres of cloth. A 90 x 170 cm bath sheet is 1.53. That is about 55% more cotton at the same weight, which means about 55% more water carried back to the rail after every shower.
Drying is roughly proportional to how much water is in the cloth and how much air can get at it. But a bath sheet does not get 55% more rail. It gets the same rail, folded over further, with more of its own surface pressed against itself. In practice the drying time does not go up by half, it often close to doubles.
Three questions about your room
Is the rail heated? If yes, a bath sheet is comfortably dry by morning and the whole question goes away. If no, keep reading.
Does the room have a window or an extractor that actually gets used? A bathroom that clears its steam in twenty minutes will dry a bath sheet in a day. An internal bathroom with a fan on a timer will not.
How many people share the rail? This is the one that decides it. Two bath sheets on a single rail overlap, and overlapping towels do not dry at all. Three people sharing one rail should be buying bath towels regardless of anything else.
When a bath sheet is the right answer
If you like to wrap yourself and stay wrapped rather than patting dry and getting straight into clothes, a bath towel will not close around most adults and a bath sheet will. If you are tall, the same applies. And if you have a heated rail or a drying cupboard, the drying argument disappears entirely and you should buy the size you prefer.
The middle option people forget
Buy bath towels and buy more of them. Two 500 GSM bath towels in rotation cost about the same as one bath sheet and one bath towel, and they will always outperform the pair that includes a sheet, because one is always fully dry. Rotation solves the problem that a bigger towel creates.
Measure the rail first
Before ordering anything, measure the rail. A 90 cm wide bath sheet folded once needs 45 cm of clear rail to hang without touching its neighbour. Most bathroom rails are 60 cm. That is room for one bath sheet, or two bath towels, and it is a more useful number than any spec on the product page.