Plant closer together and with more starts per a given area you get more sun catching for the first portion of the season. Once your plants start shading each other a lot then you get a higher stem to pod ratio than with further separated plants, BUT further separated plants need a larger grow area so you have to factor for whether your limitation is grow area, # of plants, dealing with pests (huge bushes can promote pests).
To put it another way the longer/better your growing season the further you ought to space them apart, although closely spaced plants also have the benefit of being more wind resistance by being intertwined together.