There really isn't much to choose between them. The new standard is more 32 hole, while older equipment and more durable stuff tends to be 36. Depending on how much you weigh, where you ride and how you ride, 28 might or might not be doable, but the weight difference in 4 spokes is pretty negligible anyway and if the point is to be able to ride and not walk home (and then have to fork out money for new rims), I'd suggest you go either 32 or 36. You can get more current choices readily in 32 (36's tend to be available but are sometimes special orders) but you can find a lot of older equipment in 36 and much of it tends to be cheaper (it's either quite cheap or highly collectible and thus very expensive; you'll quickly figure out which if you watch eBay for a few days).
If you want to be NJS, you have to be 36 because 32 isn't certified. If you want that extra bit of durability, 36 isn't a bad way to go. You can make any wheel pretty bling, but some folks like low spoke count on fixies because you get a few extra bling points up front. Don't be conned by the low spoke counts used in racing road or track -- those are with deep-profile carbon rims, special spokes, special hubs, and designs that you can't duplicate in a wheel you're building yourself.