Having been at both NATS and non NATS training colleges I feel I can comment. So here are my thoughts:-
NATS training is free and if you get through it then you will prob be sent to a busy unit but not always, however the instructors as was previously said are not all that bothered if you pass or fail as you will be one of around 150 - 200students at the college. No disrespect to them, but there are just too many people to get to know. This may also be the case at your unit depending on where you go.
NATS college is non profit as stated, however it is run as a business, with the customers being the units and the TATC's being products. Course lengths have been cut, flying training taken out, ops famil and the BA week all gone due to budget constraints. The money for all of this comes from a central pool which the units pay for and like any other business the college is paid to produce trainee ATCO's and so is accountable to the units.
Training elsewhere is costly and there is no guarantee of a job at the end of it. However the instructors will bend over backwards to help you, they are customer focused and like any other business this means providing your customer with a good service. Dont think that the training is any less rigorous than NATS training though. All the areas NATS assess on the ADI course are also assessed elsewhere for example Air/Ground ops, also the written papers are not multi guess as in NATS(Except Area).
To say you wont be at a busy unit is just crap, take for example Liverpool, East Mids or Newcastle, these are busy units in their own right which are non NATS. Suggesting that you will be any less of a controller is also just plain crap so get your head out your arse anotherthing. There are many non NATS trained people who are now in NATS working at busy units, maybe they should get a lower banding than the others at their unit if that is the case.
In conclusion NATS training is free, other than that it is no better or worse than any other college of ATC. You are either able to do the job or not, it doesn't matter where you get your student rating or validate it.