Try them ALL!
If you are not ready to purchase S3D (and if you are just getting started, don't do it) Try all the others.
The Prusa edition of Slic3r with the variable layer options is pretty awesome, and Cura 2.5 has a bunch of cool new features too. If you just need to do some custom supports Craftware has that and so does Meshmixer.
Those are all FREE.
I have them all and I use them all. Lately, I find myself doing more custom supports in Meshmixer for even more control over placement, and then importing that into Cura or Slic3r to use some of the other features, and then export as gcode. FLUX Studio allows direct import of gcode now, so we don't have to do the upload to the SD card trickery anymore so you really can use any of them that you like. As Jim and the others said try them, experiment. See what works for you.
Quite often, it is actually the model itself that will dictate which slicer I use as some things just lend themselves better to certain slicers. If I know I've got a lot of detail and may need multiple processes and complicated supports, I'll use S3D. Unless supports might get really detailed, then I will lay them in by hand in Meshmixer and slice in something else. If it is something simple and easy, don't overlook FLUX Studio itself with it's own built-in versions too. Very often that is more than capable of about 60-70% of what I need to print and does just fine.
S3D is great, but for the price, development has been a bit lacking, the others are catching up fast and can now do some things that S3D can't do. S3D is at a point where they need to innovate or drop the price.
As an end user, you have a ton of options to try out before you need to even worry about that, and you can actually look back through these forums and see that I held out for a very long time too and got along just fine without it. Don't get me wrong, it's great, but it's getting long in the tooth. Slic3r has Prusa driving it forward. Cura has Ultimaker pushing progress. They are not waiting around, and they are OpenSource.