Originally the Stan language used
<- following the common convention in R, which goes back to APL via S (the predecessor to R). Then the non-R users pushed back (including many of the Stan developers) and we added
= with the intention to eventually deprecate
<-. So the short answer is that there is no difference.
Note that you cannot assign values to parameters using either
=. Parameters are random and their values will be sampled with MCMC. All you can do is control how they are distributed and hence which values will be sampled.
Also, in the future please create new posts for new questions. Thanks.