River_rat's testing confirmed some time ago when some thought the BD+ and P1 were the same that, the BD+ uses a thicker media than P1, it's a synthetic blend as opposed to the P1 cellulose type media. Of course there is the cost factor to be thrown into the equation too. P1's can generally be had for < $6 at Kmart and Amazon, less when on sale at AAP with a code. The BD+ runs ~$11.50-~$13.50, one might be able to get them for ~$1 less on special.
I recently used a BD+ for two oci's but I got mine for ~$3.50 a year or two back on an AAP code glitch. I likely wouldn't pay the normal BD+ premium over the P1 for an up to ~7500 oci. If running two oci's an/or up to 15K oci then the BD+ becomes more cost effective. But if money is no object sure the BD+ is a "better" filter with more than twice the rated holding capacity of the P1, 29g v 13g. Still 13g for the P1 isn't too shabby.