The last time GMP did a presentation to our bike group, they said Datatag tended to be less good than other sorts of marking and registration (they postcode mark, and recommend recording details on immobilise).
The problem they had with Datatag was that the readers weren't widely available within the force, and so it was harder to check details on the spot. I.e. you're relying on there being sufficient reason to take the bike from the thief, and taken to a station with a datatag reader available. (Whereas they all carried UV lights for checking for postcode marking, and could access immobilise to check frame numbers &c during a street stop). Things may have changed since then though.
I'd say go for Datatag if it's cheap enough, but mark the bike too, and record frame number on Immobilise. You could also ask the local force what they do - here in Crewe, there's been a big push to get people registered on Bikeregister.com (visible marking, and a website that records bike details).