Sorry, but this is way to broad to answer effectively. The NYC public trans is very extensive so walking distance to a subway does not narrow it down any.
IMO, you really want to decide where you want to spend most of your time. Staying closer to time square leaves you in walking distance to a ton of attractions, but with our public trans, nothing is really out of reach. I think what you should think about what you want to do while here, and then ask for assistance/opinions on areas that will make spending time there easier. The city is filled with attractions from historic sites, theateres, shopping, museums, etc.
also, "not too posh", means nothing to those that dont know what you consider posh. =)