You don't lose a weapon to take sword. You get an extra attack. It is how it is modeled and their is really nothing to replace it except a strength 10 ap1 gun at bs 3 24" range.
With 25 free points there are not many options
For grinder
Warp gaze for 25
Warp sword for 25
Torrent flamer for 20
For one wound toughness 4 and lower and none 2+ save the warpsword is better.