PDW is great. Definitely use extended mags.
It plays differently than the 3 round burst pistols from previous games- at first I didn't like it, but once I got used to it and put the ex. mags I loved it. It only carries a 12 round mag, so 4 shots, which makes reloading too frequent. With mags you go up to 18 bullets/6 pulls of the trigger.
After that, I'd probably go with tactical knife or silencer, If your primary is silenced, then I'd put it on secondary too, if not, Tac knife may offer more utility. Do not skip Extended Mags under any circumstance on the PDW.