16-inch for limbing; 20-inch for felling.

The short bar is faster and easier to maneuver in tight branches. The long bar gives you more reach and can handle bigger trees without a second cut. If you can only have one, buy the 20-inch if you mostly fell. Buy the 16-inch if you mostly clean up.

Limbing is about control. A 16-inch bar lets you get the nose into tight spots, keep the saw balanced, and avoid bouncing off limbs. It’s lighter, so your arms don’t burn out after an hour. Felling is about leverage and not sticking your bar in the trunk. A 20-inch bar gives you room to cut through a 20-inch tree in one pass, and you can reach deeper into the wood when you’re back-cutting. You also get more bar length to work around lean and tension.

The trade-off is weight and power. A 20-inch bar needs a saw with enough torque to push that chain through a wide cut. If your saw is underpowered, the long bar will bog down and dull faster. A 16-inch bar on a 50cc saw is a sweet spot for most homeowners. For a pro doing both tasks all day, you want two saws or a quick-change setup.

Get the 16-inch for limbing and light felling. Get the 20-inch if you regularly cut trees over 16 inches diameter. If you only cut firewood from small logs, 16-inch is plenty. Future You will thank you for not wrestling a long bar through brush.