They are both equally good, and widespread in the literature. Go for the one that you have more familiarity with. I personally use GROMACS, since I found the setup, commands and documentation are a bit better, however, there is also good ones in NAMD. Also, NAMD has a plugin to VMD, to make it easier to use (at least for some uses).
both of them resolve motion equation. GROMACS is easier to install in a single computer or in a cluster and have a huge set of instruction to obtain a lot of properties.
Most of the previous comments are right. First, you should consider several factors:
1) What you want to calculate? (What properties)
2) How many computational resources do you have?
3) Do you have experience in MD simulations?
4) Just classical simulations, QM/MM, etc?
......
There are many softwares for MD simulations. Usually, NAMD or GROMACS are both convenient. If you are able to use supercomputers with many nodes in parallel, use NAMD, otherwise, use GROMACS. Amber probably has the most robust force field, especially if want to include simulations with DNA and proteins. CHARMM and OPLS-AA are essentially at the same level than AMBER.