Is there any reason to take the "Mentalist" class over the "Wizard Mentalist" class? The WM class seems equal to and better than the Mentalist only, so, wouldn't the WM make the Mentalist an obsolete class?

No, the Mentalist is a specialist of Psychic Disciplines, he can spend in Psychic Ability the 60% of his DP while the Wizard Mentalist only 50% in Psychic Ability  and 50% in Supernatural Ability; she is a mixed-class.

 Ah. OK. Thanks.

 Not only that the Wizard Mentalist gets his bonuses per level split between psychic and fighting abilities, while the Mentalist gets all psychic abilities (therefore will be a much better at them) I actually think the Mentalist is stronger myself, since they can pick where to spend there extra points due to getting a lot of focus on the primary.

Also WiM will always has a worse maximum psychic and magic projection than a mage or a mentalist going with full projection. But, you see, it doesn't mean WiM is crap because full projection is not always needed and there are many spells and psychic powers that don't need a good projection throw, let's say, just to affect oneself or someone/something you are touching you need a 20 in the die roll. Special mention to automatic spells, because these are sometimes thrown on oneself and may affect anybody in the area of the spell or in a further zone and in both cases there's needed a projection throw with different difficulties (see the tables of the range of projection). That's quite important when choosing WiM's spells if you decided to not to have a high base projection ability.