Coolness is one thing, but do you really have a need for a 14" SBS? I only ask this since if you have a choice between the two, I'd go with the full length M4. First being enhanced mag capacity and second being sight radius and range for blasting with slugs.
I have a 14" M1S90 - while it is cool and all, it's usefullness is really limited. I call it a 50yd and under blaster with slugs, but am also just running rifle sights on it. Also have a full size M1S90, so I have a few things to compare it with.
If I was forced to have one, it would be my full size M4, not the SBS. If you have the coin there's no reason you couldn't get both (like I did), but for just one go full size.