Actually there's a huge difference in cost -- labor cost. I only just now learned of Phil's kit. It looks like a bolt on replacement, which is nice for DIY. The Pilbeam requires actual skill to install. Also the Pilbeam is an entire pedal box, which should gain rigidity vs the stock setup with Phil's add-on. The bias bar itself is in the cockpit for the Pilbeam as well; don't know how much of a difference that makes.
If it were me, if I were racing, I'd go with a setup that follows the design of every (non-floating, non-bearing mounted) racing pedal box out there. If I were just wanting to get tuning for my otherwise non-dedicated track car, I'd look hard at Phil's.
I love the Pilbeam, but my car is 100% race car with the works.
Pilbeam's is a well designed, lightweight piece of kit. He was on the championship winning Lotus F1 team designing chassis parts, then transferred over to Lotus factory after the team folded. Can't get any better credentials than that...and it shows on his products.