Here are my suggestions based on your parameters.
The Millennium Tower (301 Mission St)
-If the southern part of the financial district is neighborhoody enough, this tower is top notch.
The Infinity Towers (Two new green towers at Spear & Folsom)
-Located by the Embarcadero, these towers are in a primo location. Although the area is not as neighborhoody as the Castro, it does have some nice places to walk to nearby.
The Greenwich (Van Ness at Greenwich)
-Not in SOMA but very walkable area, with Cow Hollow and Marina Green close by.
-Remaining units may be sparse at this point but I could certainly check for you if interested.
The Brannan (229 Brannan St)
-Built in 2001. Sellers are discounting prime units right now. Overall a well-managed, desirable building in South Beach.
Those are just a few of my suggestions. I would keep in mind the following as you search these areas:
1) Have a buyer's agent. It costs you nothing and many agents (myself included) know the inside scoops about these buildings and how much you can truly get units for. Asking prices these days are soft.
2) Don't buy anything in any of these buildings unless you're going to own for at least 4 years, preferably more (that's my opinion, anyway).
3) There are many buildings in the SoMa and South Beach. However, one must be careful because some perform very well resale wise, have better HOA's, and have less problems. You'll want to stick with the best ones and avoid the buildings that are fledgling. A good agent will know these trends.
4) No matter what building you choose in SoMa or South Beach, none will provide the same level of "neighborhoody" charm that you got in the Castro. If you want that established neighborhoody feel, I would add Nob Hill and Russian Hill to the search. SoMa and South Beach can feel a little "new". Despite this, my favorite locations out of the above are the Infinity Towers and the Millennium Tower.
5) Good luck and call or email me if I can assist you further!