FRA is my preference - you will get a limo transfer to plane at FRA but not so likely at MUC, where you will often have to walk through part of the terminal to the gate. FRA FC terminal feels more spacious and you can see outside, whereas MUC has a rather enclosed feel. But both are very nice.

If you are travelling solo to SIN , a more important factor to me than the lounge choice in Germany is the aircraft LH use. The A340 used on the MUC-SIN route only has 8 F seats, all with aisle access, whereas upstairs on the 747 on the FRA-SIN route there are 16 seats in F. This doesn't feel so private because you are likely to have another passenger next to you and if you have a window seat you may have to step over them somehow.