ok, i have discussed my personal experiences with both ecu's and i'll try to be as fair as i can once again.
the p30 is the BEST ecu for a stock/close to stock b16a. the fuel curves and timing are excellent in making the best power for this setup, and remember, a b16a from honda runs perfectly, so why fix what isn't broken.
ok, with that said, chipped p28's almost always have programs that are designed to run with 1.8's. so basically, its like running a p72 (gsr ecu) with your b16a. the timing is not correct...which is going to cause a loss in hp. second, these ecu's are usually designed for mugen/jun/spoon motors which are higher compression. this means they burn more fuel, which your stock b16a just isn't capable of doing because of its lower compression and smaller displacement.
and hondata p28's are used becasue they are most common, the fact of the matter is any vtec ecu can be used, it just so happens that p28's are the least expensive.