The less expensive solution is shortening the spring, but :
pro : less expensive
cons :
. you have to dismantle the suspension, reassembly it without the rebound spring, measuring the height of the shortened rebound spring, shortening the rebound spring and assembling again the suspension. Two dismantling/assemblies. In your case you have just to measure and find a rebound spring with the correct length.
. by cutting the spring : you increase the stiffness inversely prortional to the number of coil.
The sit of the spring is not flat, you have cutted the flat coil on one end. Generaly the both ends of the active coils are diametrically opposite in order to avoid bending, you are not sure to get this.
. If the main spring is shortening in the time, the gap increases and cannot be corrected.
Perhaps SSL has a cost but offers a clean alternative and allows for adjusting the gap in the time. SSL kit includes paired dampers and bearing. During the assembly, compression of the main spring is less than in the standard and assembly is easiest.
Another "con" is that cutting down the rebound spring so reducing the number of coils, increases its spring rate. This means the collective spring rate on rebound is even higher which may not be beneficial, i.e. too much roll resistance affecting steering characteristics. It also increases the importance of rebound spring not being in play when static to avoid very high combined initial spring rate under compression, as may happen if new main springs are fitted for example.
Above mentioned Peter Ballard's (a director at SSL) fuller explanation
here
