When using a bridle, I used to use a rolling hitch and that's what is generally accepted as the best knot for that. You can use it on rope or chain, no issues.

Last summer I switched to using short double-loops of Dyneema single-braid in a prussic knot, in conjunction with pre-made bridle lines that have snap hooks that are rated for high load. (I use the Wichard Model 2328 hook). The short loops are about 12" (doubled) and make the prussic knot quick and easy (no long tail to deal with) and the bridles snap on/off quickly, which is a nice thing while leaning over the bow rail.

Here is a picture of the Wichard hook -- I splice these into the ends 25-foot 3-strand nylon lines for the bridle, so there is a separate hook for each bridle leg. Another nice thing about this arrangement is that I can use the snap hooks to attach my bridle to guest moorings that might not come with appropriate bridles, and if there isn't enough room in the mooring eye I can use the Dyneema loop to attach to the mooring also. Dyneema is more chafe (and UV) resistant than any other synthetic line material. (I splice my own double loops, but that's for another post, if anyone's interested). I got the idea from the Shockles Line Grabber http://www.shockles.com/products/linegrabber.cfm but I use modified brummel splices, instead of the lock-stitches used by Shockles -- so my splices are almost as strong as the rated line strength.