I tried many different strategies. Usually my location set up is a lot more than what you mentioned, but sometimes I go with a body, one speedlite, pocketwizards, umbrella and a stand. Sometimes I go with Elinchrom Ranger Quadra system with multiple camera bodies and lots of lenses. So, I have one large and one small Lowepro Pro Roller case. Basically, I put everything other than stands and umbrella in one case. Then the stands and umbrella go into a Hakuba case. This way, I can carry everything without an assistant, although I usually carry reflectors, diffusers and sometimes backdrop anyway, so I need extra hands. For low budget shoots I ask my clients to help, but either way, it is best to use a big case and put everything in it. Good location cases do not double duty as a universal storage case.
I also like taxi a LOT better than driving myself or taking subway when shooting in Boston. Buses are okay but it really depends. If I were given absolutely no option other than subway, I'd consider downsizing everything and use a backpack rather than rolling case.
Another thing. If you are shooting outdoor in daylight, you probably find very little use to carry grids and snoot, especially with Speedlite. It's more useful to have an efficient metal narrow angle reflector.