As far as I am aware, the short side rail (SSR) mount for the K98 was developed in the mid 30's for use by German police and paramilitary. It saw service during WW2 between 1941 and 1945 once it had been improved with locking screws and tapered pins to keep the mount from working loose under recoil. The scope (usually 4x Ajack, Zeiss, Hensoldt, Khales) was held in place by machined steel rings with two lock screws. The SSR was used most notably by the SS, who used many Gew 98 receivers reworked into k98k rifles. The SSR could be mounted on any standard Mauser 98 receiver due to its contoured shape.
During the later stages of the war (1944-1945), Gustloff Werke started to manufacture the Long Slide Rail (LSR) variant of the K98 sniper rifle. This new LSR mount incorporated locking screws, as well as a spring loaded locking latch which prevents the mount from sliding free should the main locking lever come loose. Tapered pins through the mount and receiver were also used, but not in every LSR rifle. The major difference, however is that the LSR mount was designed to be attached to a specially made receiver that was enlarged and machined flat on the left side, as opposed to the standard rounded action on most Mausers. As a method of speeding up production later in the war, the original machined split ring mount was replaced by metal spring bands, and a recoil ring was usually soldered onto the scope to keep it from sliding forward due to recoil. These mounts were used on late-war models of rifles produced by Gustloff Werke (bcd), J.P. Sauer (ce), and Mauser (byf).
All of this information I learned through Peter Senich's book "The German Sniper 1914-1945". It is very informative and I'd highly recommend it if you're interested in the subject or are trying to make a K98 sniper clone, or if you are swimming in money and are going to buy a real one. If that's the case, I am super jelly.
As to which is best, I'm sure they all served their purpose very well if built properly. Germans were (are) not known for building crap. however, it is widely accepted that the turret mount system is usually the most sturdy form of K98 sniper designs.
