Depends on the person tbh. Most people ideally need a horse that is something between the 2 extremes as an overly sensitive horse is likely to scare a novice by over-reacting to aids they don't realise they are giving whilst an especially dead to the leg type will only teach them to reach for the whip rather than how to apply the aids correctly. Someone really struggling with their balance may be better initially on a steadier type that won't be unsettled by them until they gain confidence (perhaps on the lunge until their position improves?) whereas someone who is naturally a quite and subtle rider or someone less physically able may be better suited to something a little more responsive. That's not to say that this shouldn't be mixed up a little as the rider progresses though as every horse will show / hide different strengths and weakness
Thanks for your thoughts. I was wondering as I can see the benefits both ways. If someone learns on a super responsive horse, I guess they will really struggle to get anything out of the less well schooled one - perhaps the other way round is better, as you can soften your aids once you have learned how to apply them....