I can't speak to 7 spd because I only have 9 spd, but I prefer SRAM for an odd reason. I have always found Shimano cassettes more difficult to get on and off. For reasons I've never understood, I have tended to have trouble getting lockring tool to seat properly in shimano lockrings but have never had any problem with SRAM. That said, the difficulty was just a minor annoyance (but enough that I usually go with SRAM), and I have not noticed a difference in durability or performance between the two cassettes.

Regarding the choice between 12x32 and 13x30, there is no reason to choose cassette just based on what LBS happens to have in stock. Its easy enough to find cassettes with gearing to meet your needs, so whether you go with Shimnano or SRAM, if neither the SRAM or shimano that the LBS has meets your needs, shop around for the set up you want (probably available any number of places online).

-- After a quick search, I may be wrong about availbility of gearing options. I am not seeing 7 Speed SRAM in anything other than 13x30 but more options with Shimano so perhaps thats a reason to lean toward Shimano.