Is there a difference?
I have a Cateye HR10 that appears to be a Radio signal model - it is generally reliable but often will give me wacky readings (like "0" when I am pretty sure I am still breathing, and "225" when I haven't even taken the bike off the car rack yet). Sometimes the cause is obvious (another HR monitor or radio towers nearby), sometimes not.
I just got a new Sigma Onyx Classic for Xmas and it says it is "Digitally Coded." I haven't tried it yet (was thinking of selling/regifting since I have the Cateye already) - would this be a different technology from Radio signals, and would either be more accurate/reliable/steady than the other?