I have a Fidelity Designs Reference sub-platter on my Planar 3 and I think it's excellent. It's very nicely made, and it made a significant improvement to the sound - I think that molded plastic sub-platter is probably the weakest part of Rega turntables.
The only thing I didn't really like is having the platter sitting directly on the metal surface of the sub-platter - not for any rational reason I can think of, mind. But I did have a couple of little damping rings from SRM/Tech, so I use one of those as well (with an SRM/Tech acrylic platter).
Bottom line for me is that I suspect there's very little difference between different makes of alloy sub-platter, but the price of the Fidelity Designs ones make them good value.