Buying towels as a set is almost always the better move, because the alternative is what happens in most houses: a drawer full of orphaned bath sheets in six different colors and a hand towel nobody remembers the origin of. A matched set forces you to actually think about what you need, once, instead of grabbing one more towel every time you pass a sale rack. The exception is if you live alone and genuinely do not care whether your towels match, in which case none of this matters and you should stop reading and go buy whatever is cheapest.
Where sets go wrong is in the padding. A lot of them bulk up the count with washcloths you will never use and a bath mat that wears out in a year, so look at the actual composition before you buy, not just the number on the label. In the shop, rub the towel between your fingers and pull gently at the stitching on the hem and the loop, because that is where cheap sets fail first, usually within a handful of washes. On delivery day, check the weight is consistent across every piece in the set, since some brands will ship you one dense, decent bath towel and three thin ones to hit a price point, and you will only notice once they are already in rotation. Stick to grams per square meter if it is listed, anything well under 500 is going to feel thin no matter how nice the color looks folded in the photo. For color, plain white or a pale neutral is the easiest to live with long term, because bleach is an option and you are not trying to match a discontinued dye lot when one towel finally gives out. Deep solid colors look better for longer than anything patterned or striped, since patterns show fraying and pilling faster than a solid ever will. Avoid very dark colors in a shared bathroom if you also own white bath mats or light grout, because dye transfer is real and it is not coming back out.
A good set earns the extra cost over months, not days. The first wash matters more than people treat it, since a lot of shrinkage and most of the lint happens right there, so wash new towels on their own before they ever touch anything else. After that, wash on warm rather than hot, skip fabric softener because it coats the fibers and kills the absorbency that you paid for, and tumble dry on low with a couple of dryer balls if you want them to stay fluffy instead of going flat and papery. A decent cotton set will soften with age instead of thinning out, and it is usually the cheap ones that go stiff and start pilling around month four, which is the real tell you bought wrong the first time.