this is right in my wheelhouse....quick rundown:
Pacific Beach has a college atmosphere that skews younger 21-25...drinks are cheap, but the crowds are a lil immature, and entitled so to speak. The best spots in PB are Alehouse and Barwest. Barwest is club like, but the dress code is crazy relaxed. Only drawback is that BW is tiny.
If you're looking for a more traditional club spot, then you wanna hit downtown/gaslamp. There is Fluxxx, which is our trendy spot of the moment...its almost exclusively house music oriented, and its where all the broads and douches who wanna be seen go. There is Fsix, which is geared more to a hip hop crowd, but that spot can be hit or miss. My personal favorite is Ivy, which is huge (multiple levels), fairly diverse in crowd, and always draws a good selection of females. Stingaree and Sidebar are also good options.
If you're looking for hipsters, then you prolly wanna hit the north park area, which is kind of between pacific beach and downtown. North park is teeming with the hipster types, skews older 25-30ish, and is pretty racially diverse. Great selection of music --- you're liable to hear anything, mostly centered around the 90s. Best spots are True North, and A.C lounge.