I have the SONY 90 and find it very sharp. In low light underwater it does often hunt for focus, but with a strong focus light this is not an issue. While some (above water) comparisons find the Tammy to be sharper the differences seem to be minimal and even miniscule stopped down to F16. The SONY has some advantages in that it has image stabilization ( Tammy does not, but most SONY bodies have IBIS) and stops down to F22 ( Tammy is limited to F16, a limit that should be fine for most applications). The biggest advantage of the Tamron is price, although a good used SONY 90 can be had for the same price as the new Tamron. The older SONY 90 is due for a ver II upgrade, although I haven't seen any rumors of this. Photographers not shooting macro are more focused on SONY's excellent 85mm which was recently upgraded so it might be a while before a new 90 is released.
