Automatic is most convenient and costly. It works by sensing the increase in rpm as the line wears and gets shorter, then releasing line until the rpm return to normal. However it's advanced, the line eventually runs out and needs replacing. Straight-shaft trimmers offer the most power, comfort, and flexibility. Their shafts are longer, which gives them more reach for easier cutting under shrubs and fences.
Reduction gears lower the spinning speed and thereby increase the power to the line.
Others rely on flexible drive cables in plastic liners. Both are reliable, but solid driveshafts are smoother and more durable. If you plan on cutting heavy brush with a blade, favor units with solid driveshafts. Curved-shaft trimmers are the lightest and most economical. They deliver power to the cutting head by a flexible-cable driveshaft.