  The class of mixed Tsirelson spaces is an important source of examples in the
recent development of the structure theory of Banach spaces. The related class
of modified mixed Tsirelson spaces has also been well studied. In the present
paper, we investigate the problem of comparing isomorphically the mixed
Tsirelson space T[(S_n,\theta_{n})_{n=1}^{\infty}] and its modified version
T_{M}[(S_{n},\theta_{n})_{n=1}^{\infty}]. It is shown that these spaces are not
isomorphic for a large class of parameters (\theta_{n}).
