It's probably a wash. A one way FRA-SIN in April will run you about $550 on something nice like Qatar, Etihad, etc. You may be able to get down to $450 with double connections.
Round trip with a random return date in Oct would be around $700. I'm not sure about the exact change rules, but if it were United, it'll cost you around $250 to change the date. If you want o go this route you'd have to study the fine print for every option. If the change fee is less $300, your probably better off doing the round trip.
Main advantage on the round trip @700: it included the non-stop from on Lufthansa which saves you a connection. If your departure point is not FRA, that wouldn't count.