Currently use full leather collar for obedience classes.I am considering changing to half check in the short term.My 6 month medium sized hound pup is remarkably strong.Will this be both an 'effective' and 'kind' collar to use.

Our classes only allow half check collars unless a dog has a particular problem. You'll find they are far more effective than a full collar. I use them (on 2 very strong GSDs) for everything because a dog is far less likely to be able to "escape" from a half check than an ordinary collar. It has to be the right size (most of them are adjustable)  obviously, and make sure you get one thick enough for the dog's size & strength. Nothing unkind about them at all.

Nothing but a flat collar or harness allowed at my classes.

Our class also stipulates that check or half check collars aren't allowed in our sessions.

I do use half check fabric collars for day-to-day walking though, ever since I walked past an over protective swan.  I looked down to praise my pup on his bravery only to find myself talking to an empty collar.  He was sitting patiently back at the start!

Some dogs find it very easy to slip a collar over their heads so the martingale, half-slip types are ideal.  Do be careful with fit and if you have a look around online you should be able to get some guidance inc video.  I'm not sure how effective they are with real 'pullers' though.  It's certainly preferable to deal with pulling via training first and foremost - try adding some extra training off lead heel walking in your garden.

Ditto - my old club when we started 9 years back said half check, but by the time I got the next puppy had changed to flat collars, and my new club is the same.

As we don't use collar corrections in my classes at all, I don't really care whether people use a normal or half check (provided the half check is adjusted correctly and doesn't choke the dog when closed up). A half check won't make any difference unless you use collar corrections to teach the dog that the noise of the chain means discomfort (in which case it won't be "kind")... otherwise the dog won't be reacting any different in either collar.

I only recommend half checks/martingales as such for dogs that can slip a normal collar (e.g. sighthounds).

Also they are invaluable for dogs with thick coats and ruffs. 

With a  thick coat a flat collar will mark/wear the coat, and also is rarely done up tight enough to prevent it being slipped, as it then becomes difficult to move it round and find the D ring to attach a lead, with a half check it is easy to pull the chain out fo the coat to attach the lead.

it also means that at rest the collar lies comfortably low down on the neck

As the original poster didn't say anything about the reason he was considering swapping being that his current collar is slipping over his dog's head, and wants to know whether a half-check would be 'effective' for a 'strong' dog, I understood that he's interested not in its secure qualities, but in its ability to administer corrections...

The thing about half-checks which most people don't realise is this (sorry if this is teaching my granny to suck eggs for some of you, but for many it will be interesting):

Decades ago, when most obedience classes used choke chains to train dogs, the sound of the chain running through the loop signalled an impending correction to the dog.  The message was "noise = choke chain jerk coming". 

In those days, people could then switch to half-check collars and still achieve results because the dog heard the same sound of chain running through a loop.  The dog then quickly performed the behaviour required (stopped pulling; stopped doing whatever was undesirable) and believed that he had escaped the choke chain correction by performing the behaviour so quickly - this in itself was very rewarding to him.  (To avoid a correction.)  This is avoidance training (same as ecollars in the US use) and it is extremely effective (whether ethical or not is another debate).

With the exception of dogs slipping their collars and ruffle-haired dogs etc, and talking from a training perspective:  The reason why it makes little sense these days to use half-checks is because most people using them have not come from a background of choke-chain training and leash jerk corrections.  So when the dog hears the chain sliding through the loop on a half-check, it doesn't signal anything at all impending and it has no effect in making the dog perform the required behaviour. 

Hope that makes sense.  In short, the function of a collar is to keep your dog physically safe and to put your address tag on.  Moving on from those practicalities, as far as training goes, there are only rewards and punishments involved in manipulating a behaviour.  And any collar is not administering rewards, that's for sure....  And if it's not administering either rewards or punishments, then it is not effectively manipulating a behaviour.  Therefore either it's working by punishment (so is not the 'kind' solution the poster wanted) or it's not going to affect behaviour at all and you should save your pennies!