I recently did a search with all 3 of the agencies, to check if some things had dropped off (I have an account with Noddle, and used the Experian and Equifax free trials).
The Experian one IMO was the most useful and up-to-date, the Equifax and Noddle ones were quite similar in terms of what they showed, with the Noddle one showing things which were over 6 years old, and therefore should no longer have been on there.
The Experian one also highlighted each of the "problem areas" with recommendations on how to improve them - most of which were pretty much common sense, but may still prove useful.
In terms of scoring, they each use their own method of scoring, so you may find it varies quite a bit (my Noddle and Equifax both showed quite a poor score, whereas the Experian one was quite good). However, since each lender also scores in their own way based on their own criteria*, the credit agency's own score isn't particularly useful except as a broad indicator.
So basically, in my experience, Noddle is fine for getting a rough idea of where you are, but if you were going to pay for one, I'd recommend Experian over Equifax.
However unless you want all the realtime monitoring etc. you'd be better of just doing one of their
one-off £2 statutory reports
Even if you do one every month, it will still cost you less than 2 months' worth of subscription!
* E.g. a high APR credit card aimed at people with poor credit may actually be more likely to decline you if you have a good rating and always pay off your balance in full, as they probably wont make any money from you
