I have the KSP40 , which is the same saw without the short rigid rail, it uses the flexi rail, its an older model .
So - rails you can only use the flexi rails with these small saws, and they only come in one length.
The KSS300 should be with the short rigid rail that turns it into an effective portable mitre saw.
The most important consideration is the intended use , these are tiny saws and whilst they will cut up to 40mm they are not suited to dimensioning 18mm ply or similar all day - they have a 120mm blade , they are perfect for flooring , trim, skirting , they are very light , incredibly well made and have no competitors , typically used by shop fitters and such.
The KSS400 is a better bet for bigger jobs - it has the 160mm blade like the MT55c plunge saw.
The K series saws have riving knives (sprung) so are better suited to cutting off rail than the MT55c , the dust collection of the K saws is naturally less impressive than the MT55c as the blade is not fully enclosed.
I got my KSP40 used , fitted a new rubber strip to the flexi rail and new blade - Trend do an affordable version. I wanted something light as I already have an MT55c 110v so it weighs an extra 20kg... (no choice it was bought used)