530d is a 218bhp engine as standard a 330d is 231bhp.
both remap well, but the 3 series would be quicker as it's lighter and sportier.
the 535d however is a mix of turbo's to get shifting with little lag and the second turbo kicks its arse up the revs, they do remap well also.
i have been eyeing up 330d's as well, and a standard one pretty much keeps up with my 330ci, probably not to the top end, but not far off in terms of acceleration to 100mph.
best just to have a word with James at MSD, his is up for grabs, cheap tax, cheap to run and circa 250bhp.
best buy if you have the dough.