NLMIXED

I am a SAS user and I am using PROC NLMIXED to fit growth curve model with
Gaussian data. Here is my question. I have the SAS for Mixed Models book
and also the PROC NLMIXED document.

To fit growth curve model for Gaussian data, each of the references have a
different non-linear equation:

SAS for Mixed Models:

Yij = ( ( B1 + ui1) / [ 1 + (B2 + ui2) * exp( -Xij / B3) ] ) + eij

PROC NLMIXED:
Yij = ( ( B1 + ui1) / [ 1 + exp( -(Xij - B2) / B3) ] ) + eij

I am not sure what would be the difference, if any, in fitting these two
models separately for Gaussian data because both are mentioned
as 'Logistic Growth Curve Model'.

Even if I assume there is not random effect for B2 in the equation
from 'SAS for Mixed Models' I am not clear about the difference.


I appreciate your help.

Zhu.