NAMD better scales on the big number of threads. GROMACS is more efficient per thread. In addition NAMD have a drude polarised force field by default and GROMACS is a bit easier to use. Anyway, the differences are small and both are very good the molecular dynamics software.
Gromacs has inbuilt commands for most of the analysis which namd don't have. Hence for namd u'll have to write your TCL or python script for analysis. Gromacs is faster than namd. Still both are useful softwares.