Without question I would go with the OPPO. It has honestly been the best DVD/BD Player that I have ever owned. Little things like the Disc Loader developed in conjunction by the Tohei Group of Japan just make for a wonderful experience. While I still love my Pioneer BDP's, I am not as big of a fan since the BDP-09 and 51/05 era as they were the last that were Built in Japan and actually made by Pioneer. All since have been manufactured by Sharp.
While the 53 is definitely a step in the right direction and uses the same Marvell Qdeo Video Processor as the BDP-93 and 95, OPPO is almost fanatical about revisions and getting every last ounce of performance as evidenced in a practically non stop stream of Firmware Updates. While the 53FD is the most appealing BDP from Pioneer since the 09FD, I would still go with OPPO.
I think if you read the professional reviews it becomes evident that the Oppo is the best option... I have to admit, I, like Jungle jack, own an Oppo93. Everything from the packaging to the customer support... To the build quality... And the usability.... Is really top notch. It's also dead quiet. Great little machine - albeit expensive.
I have zero hands-on experience with the pioneer ... Haven't actually read any reviews on the unit. But, while I did my research the Oppo - time and time again - popped up to the top-o-the-heap. For me, it was a no brainer.