Due to the above comments about lighting, and I certainly agree with the comment on the use of lasers, I would recommend the X400U...
I thought about this same thing just a little while ago...I wanted a laser, and while the IR capabilities would be nice, I don't don NV at work...so it is extra weight that I really don't need. The X400U gives you the visible laser when you want it, and the light when you need it. I like the idea of the laser for the unorthodox shooting positions one may encounter. It doesn't add much weight over the X300U, if you knew you wanted a good light source, but it gives you the added comfort of the laser.
Realistically you need to assess what your use with the laser will be....even a standalone makes sense if you are going to be using it at distance and/or for durations of time...the Surefire X400U is limited in its battery life (almost a 4th of the laser devices OTAL).