I recall reading somewhere that body painting is usually done at low pressure to avoid any possibility of embedding paint particles in the skin. If that's true, then a bottom feed would be undesirable, as they stop working at pressures below 20psi or so.
If you want one brush to do everything, I'd say get a side feed, as that will give you the color capacity of a bottom feed, with the low pressure ability of a top feed. You won't be able to do low pressure and large capacity at the same time (large capacity side bottles emulate a bottom feed, and thus need more pressure), but you'll be able to hot swap colors like a bottom feed while using low pressure like a top feed.
TBH though, I'd recommend two brushes: one specialized toward broad coverage, and one towards detail. Set them up on a manifold (two hoses) instead of a single hose and/or quick connector. I'm a model maker rather than a body painter, but I have a thing for large scale and 1/1 stuff, so that's what I do.
Whether hot swapping color cups is really faster than rinsing a top feed cup depends on brand/model. Some bottom/side feeds flush clearer than others.
Are you looking at doing studio work, or more fairgrounds/boardwalk type stuff? The former will give you a bit more room to breathe and go to town with the details time-wise, but the latter selects hard for speed, so which one you're doing may have a large impact on what kind of setup is optimal.