The two are not very similar.  T. maculatus is smaller (usually about 2
inches, while T. niloticus reaches over 5 inches).  T. maculatus has a beaded,
or granular texture, while T. niloticus is smooth.  The pattern in T.
niloticus is fairly constant, irregular dark red axial bands, which become
lighter (pinkish) on the base.  T. maculatus is much more variable, often with
irregular mottling of browns, greens, and reds.  The sides of T. niloticus
(from apex to shoulder) are straight, (in very large specimens, somewhat
concave, due to outward growth of the last whorl), while in T. maculatus they
are somewhat convex.  They are both wide-ranging Indo-Pacific species.
 
Paul M.