You need to think of four units when thinking of the Kustjägare and FSK, the Fallskermjeger and the Jäegere. They cover different areas of the same job, you just pick the combination you want.
Kustjägare and Jäegere both have fast helos, so they're your helo insertion troops. They have medium-range, average fire-rate AT weapon so they're fit to every situation. They both lack combat capable transports so they have to be unloaded before contact.
Jäegere has a better MG but in this case, the meaningful difference is the very good optics stat. Kustjägare has 15-men survivability and more units per card so it's a pick between intel and longevity.
FSK and Fallskermjeger both have the Eryx, so they're great area denial tools but will suffer in forests vs cheap firesupport. They lack fast helis so forget about early insertion. They do however have the XA-186NO transports so they can drive more or less straight into combat.
The FSK has around 50% more anti-infantry firepower and slightly better Eryx accuracy(due to elite training) but the Fallskermjeger is cheaper, more units per card and has 15-men survivability. It's a pick between firepower and longevity.
If you find preserving your units and resupplying them into full health difficult or too much work, stick with the 15-men variants that have more units per card.
If you know you can preserve them, the Jäegere and FSK give you better results. They are the "better" choice in theory. However, in theory, you also never get hit by bombers, artillery, mortars or flanking enemies...