The glock 17 has fewer parts to assemble as opposed to the glock 18. The only notable thing on the glock 18 is that it is full auto, but normally it is not necessary for a gun as basic as a glock. Also it is easier and to obtain a glock 17 than a glock 18.
Both the Glock 17 and Glock 18 are reliable, simply to use handguns in 9mm. The Glock 17 is semiautomatic. The Glock 18 has a selector switch that allows it to fire in either semiautomatic or fully automatic modes. A fully automatic hand gun has limited utility. It only holds 17 rounds, which will be expended quickly leaving the user out of ammunition. If it had a three round burst function, that would be preferable.