Right up front, and this is only my opinion, my choice is Scream Machines. I have always found the interface easy to use, and if you are going for "playability" it is great. I will concede that graphically, it is not on the level of No Limits. I own both because they are roller coaster sims (heck I still own Ultracoaster), and I like them both, but for the times when I just want to jump in and begin building something, SM is my choice. For me, I just find it easier to work with whereas NL can overwhelm me at times.
Also, many of the tracks I find here are fantastic and fun to ride. However - and again this is only for me - I am a purist when it comes to building coasters, and have chosen to not use tools on any of my coasters. Are the coasters perfect? No way! But I enjoy the challenge of trying to get that curve, helix, or inversion "just right" on my own...and Scream Machines lets me do just that.