Check out the accuracy ratings and ease of use for both these items. A $40 refractometer is a waste of money you would need to spend 4-5 times that amount to get anything as accurate as a floating hydrometer. Floating hydrometers are used as the gold standand at university labs worldwide for measuring SG.
This one lists at $109
Vital Sine Refractometer
* Largest and sharpest scale display we offer
* Calibration ring allows very easy calibrations
* Durable, all-metal design
* Rubber hand grip eliminates transmission of hand heat to reduce errors
Features the largest, sharpest scale display for the most accurate salinity and specific gravity readings available (±1.0% salinity and ±0.001 specific gravity). Easy, no-tool calibration, with automatic compensation for shifts in ambient temperature (from 10°- 30°C). Compact and portable for use anywhere. Metal housing and rubber hood protect unit and prevent light from entering eyepiece. Features built-in calibration ring, no-roll stand, rubber grip to insulate against hand heat, storage case, instructions, and more. Range: 0 - 100% salinity, 1.000-1.070 specific gravity.
This one lists at $24.95
High Precision Hydrometer
* Super accurate floating hydrometer measures specific gravity
* Precision crafted by hand - needs no calibration
* Marked recommended range for specific gravity simplifies testing
Precision floating hydrometers handmade by experienced craftsmen in Germany. With a maximum deviation of 0.0005, the Tropic Marin Hydrometer is an indispensable tool for scientists and serious marine hobbyists alike. Easy-to-use and accurate hydrometer never requires calibration for consistent and exact measurement in your saltwater aquarium. Specific gravity scale measures from 1.018 to 1.028, with a marked recommendation range between 1.020 to 1.023. Accurate to ± 0.0005 at 77° Fahrenheit. 1" diameter x 13-1/2".