Spindle sander vs belt/disc sander: what each is for, per the makers

What is an oscillating spindle sander for?

WEN says oscillating spindle sanders are popular with woodworkers who need a sanded finish on outside or inside curves, and are generally benchtop machines. (WEN Products)

WEN explains that they use a series of differently sized drums, suit projects with inside (concave) curves, and move the drums up and down while spinning, hence the name oscillating. (WEN Products)

Taikra doesn't rank or test tools; every figure here is from the maker's own page or manual, linked.

Spindle sizes and oscillation

RIKON lists spindle diameters of 1/2", 3/4", 1", 1-1/2", 2" and 3", a 4-1/2" spindle length and 2,000 RPM for the 50-320, with a 5/8" oscillation 58 times per minute. (RIKON Power Tools)

WEN lists 58 OPM, 2000 RPM and a 5/8 inch stroke for the 6510T, which ships with sanding sleeves from 1/2 in. to 3 in. (WEN Products)

What are belt and disc sanders for?

WEN describes belt sanders as using a long, wide belt to quickly remove lots of material or flatten relatively large areas, and notes that benchtop and floor-standing models are often combined with disc sanders. (WEN Products)

WEN says disc sanders are generally benchtop machines that are good for smoothing end grain in wood or light metal shaping. (WEN Products)

Belt and disc sizes

RIKON's 50-122 pairs a 6" x 48" belt at 1,836 SFPM with a 10" disc at 2,510 RPM, on a 1 HP motor, and its belt tilts 0 to 90 degrees. (RIKON Power Tools)

WEN's smaller 6515T pairs a 1-by-30 inch belt with a 5-inch disc, at up to 3160 FPM for the belt and 3450 RPM for the disc from a 2.3A motor. (WEN Products)