Twin is 38" × 75". Twin XL is 38" × 80". They are identical in width and differ only in length, but that five inches drives three separate decisions: whether a taller sleeper's feet hang off, whether standard twin sheets will fit, and whether two of them can be paired into a split king on an adjustable base.
THE HEIGHT RULE. A mattress should be at least four to six inches longer than the sleeper. Twin at 75 inches (6'3") works comfortably up to about 5'9" and is fine indefinitely for children. Twin XL at 80 inches (6'8") works comfortably up to about 6'2", which is why it is the standard in college dorms — universities buy for the tallest plausible student, not the average one. If the sleeper is over 5'9" or is a teenager still growing, twin XL is the safer purchase even though it costs slightly more.
BEDDING IS THE HIDDEN COST. Twin sheets will not stay on a twin XL — the fitted sheet pulls off the corners within a night. Twin XL sheets, however, will fit a twin with some slack, so if you own twin XL bedding already it will work on both. Twin XL sheet sets are also less common in retail stores and carry a small price premium, roughly $5–$15 more per set, and the selection of duvet covers and comforters is narrower. Comforters are more forgiving: a twin comforter usually covers a twin XL adequately because the extra length hangs at the foot.
THE SPLIT KING FACTOR. Two twin XL mattresses side by side measure 76" × 80", which is exactly a standard king. Two twins side by side measure 76" × 75", which matches no standard size and leaves five inches short at the foot. This is why every split king adjustable base setup uses twin XL, and it is the strongest reason to choose twin XL if there is any chance the mattress will later be paired for a couples' adjustable bed.
ROOM FIT. The five-inch length difference rarely changes what fits in a bedroom, since width is the constraining dimension in most layouts. Where it matters is small rooms with a door swing or a radiator at the foot of the bed, and RV or tiny-house bedrooms where every inch is allocated. Measure the foot-of-bed clearance before assuming twin XL drops in.
PRICE DIFFERENCE. Twin XL typically costs $50–$150 more than twin in the same model. Frames run $20–$60 more. Sheet sets $5–$15 more. Over the life of the mattress the total premium is usually under $250, which is small relative to buying the wrong length and replacing it.
WHEN TWIN IS THE RIGHT CHOICE. Children under about twelve, bunk beds (most bunk frames are built for standard twin), trundle beds, daybeds, small guest rooms, and any situation where you already own twin bedding and the sleeper is short. Twin is also the easier size to find in stock and in budget models.
WHEN TWIN XL IS THE RIGHT CHOICE. Teenagers, adults, anyone over 5'9", dorm rooms, guest rooms that host adults, and any bed that might become half of a split king. Twin XL is also the standard for single adjustable bases, so if you want an adjustable frame for a single sleeper, twin XL has far more model availability.
THE DECISION IN ONE LINE. If the sleeper is a child in a bunk or trundle, buy twin. If the sleeper is a teen or adult, or there is any chance of a future split king, buy twin XL and accept the small bedding premium.