score:4
It all depends on exactly when you are going to travel. There is a bus taking 25 minutes from Frankfurt Airport to Darmstadt Hbf (central station), but since the bus usually only departs once an hour you might be sooner in Darmstadt if you use a train connection or bus/train combination via Mainz, Neu Isenburg or Frankfurt Hbf (some 45 to 60 minutes).
You can find suitable connections on the German Railways' homepage.
Upvote:3
S-Bahn is an urban commuter train, most stops are just a few kilometres apart. Typically they depart every 10 or 20 minutes, depending on the line and the time of day.
RE is a regional commuter train, often stops are a few dozen kilometres apart. Typically they depart every 30 or 60 minutes.
The RE will be faster, but not if you have to wait for it. The S-Bahn might require you to get several connections, which can be inconvenient for foreign travelers.