The first decision is style. A manual pump is cheap, quiet, and portable, and it is enough for occasional relief or a missed feed. A double-electric pump empties both sides at once and is the standard for anyone pumping regularly. A wearable pump is worn under clothing and lets you move, cook, or work while pumping, but it holds less milk per session and can be harder to position. Many families own one of each: a double-electric for home and a wearable for the office.
The second decision, and the most important, is flange sizing. The flange is the funnel that sits against the breast, and the tunnel diameter determines comfort and output. A flange that is too large pulls in too much areola and causes swelling and pain; one that is too small rubs the nipple and reduces flow. Most pumps ship with a 24 mm or 27 mm flange, which fits many but not all bodies. Sizing kits and third-party flanges are inexpensive and often the single biggest upgrade you can make.
The third decision is closed versus open system. A closed system has a barrier between the milk and the pump motor, so milk cannot reach the tubing. An open system does not. Closed systems are easier to keep clean and are the better choice for anyone pumping frequently or sharing a pump.
The fourth decision is suction and cycle control. More suction is not better; comfort and rhythm matter more than peak vacuum. Look for adjustable suction levels and a let-down mode that starts fast and shallow before settling into a slower, deeper pattern. A pump that only has on and off is harder to use for long.
The fifth decision is noise and portability. Traditional double-electric pumps are not quiet, and a loud pump in a shared office is a real problem. Wearables are quieter but still audible. Battery life, the weight of the motor unit, and whether it runs on a wall outlet or a power bank all affect where and when you can pump.