What is the difference between t=n*Ts sampling vs impulse train sampling?
I cannot recall a meaningful difference between the ideal impulse train sampling and the uniform sampling relation indicated by the expression $t_n = n T_s$.

Given a continuous-time bandlimited signal $x_c(t)$, when you sample this with an ideal impulse train $\delta_{T_s}(t) = \sum_k \delta(t - k T_s)$, then the relation between the obtained discrete-time sequence $x[n]$ and the continuous-time signal will be given by :

$$ x[n] = x_c(t_n) = x_c(n T_s)$$ 

Hence, the latter expression is a consequence of the former operation.