DA1855 and DA18135 are pretty close to one another in terms out their optical quality. DA18135 comes with DC AF motor that's smooth and quiet thus good if you want silent operation.
If you ask me, DC motor although is a subset of SDM is infact a far more refined motor. During focusing, you won't feel th e optical element movement of DA18135while SDM, you can still feel some.
The SDM series is of course optically far superior than the standard/ WR series but you got to pay extra.
If you do not need the WR capability, DA1770 is many folks' choice.
If you need WR. DA18135 is a good call
If you have extra cash DA1650 is the choice
If you are poor man like me, DA1855 will not fail you.