I would pick SM over HR for many reasons. More track styles, more editor options, fewer bugs, better support, etc. Also, HR doesn't offer any terrain options. Scream Machines, despite what many people will tell you, has exactly the same type of editor as No Limits. If you've gotten the hang of the NL editor yet, then you shouldn't have any problem catching on to SM.
As for SM and NL comparisons, it's all a matter of preference. Both sims have their shortcomings, IMHO. I still like both of them though.
Hope this helps!