Creatures Caves

The difference is just that a stimuli would have to be felt by the creature to be learned, while the instinct kind of gives them an idea that when they are in a certain situation, a certain behavior will affect a drive, and also tells them if that action is good or bad... But instincts can be ignored if experience disagrees with them and will be reinforced by stimuli which agree with them. Stimuli directly cause learning. Instincts are more like suggestions. The best way to think of instincts is a voice in their head suggesting or discouraging an action when a certain circumstance comes up... how loud the voice is depends on how strong you make the instinct... whether or not they follow the suggestion will be based on what they've learned... In general, they will tend to go along with the strongest instinct they have that fit a situation, unless they can think of something will work better based on their experience.

In your case, if you used a stimuli, but did not add an instinct that pushing plants reduces hunger, then the norn would be likely to naturally just obey its other instincts about food... and may never try to push a plant at all, much less when it's hungry... if it does randomly decide to push a plant, it may not be hungry when it does it, so it still may not learn that it reduces hunger very well on its own. But, if it is coaxed to push a plant when it is hungry a few times, it will eventually learn that it will satisfy its hunger that way, and will begin to use them more... Still, it would kind only have a weak correlation in its mind at first until it has pushed plants a lot of times, and probably prefer other food sources most of the time because they have strong instincts about those food sources...

If you had an instinct but no stimuli, plants would not reduce hunger for them, but they would still have a tendency to try them, because they still will keep having dreams that it pushed a plant and it reduced its hunger. If you had a good stimuli that made it reduce hunger, then the instinct will get reinforced, and it will be more likely listen to the suggestion of its instinct. If you had a punishing stimuli (that made it increase hunger, for example... actually I think just pushing anything increases hunger a little, so you might not have to make any change to stimuli to get this effect...) then it would eventually learn to ignore its instinct (man, I had the stupidest dream last night that I ate a plant...)

Those are just the basics though, now to the real meaty differences:

Instincts can be a lot more versatile in one way. Instincts can associate with the state of any neuron in its brain, or even multiple neurons at once, including actions, objects, drives, concepts, or anything about its situation, and suggest actions for any possible combination of those. So you can make them very specialized and kick in under very specific circumstances. It will suggest to the creature that some action will be rewarding or punishing in that circumstance, and which drive it will reduce (but each instinct can only be associated with one drive). Their learning based on instincts is a bit more complicated, because some of the stuff going on in their brain is not sensed consciously by the creature, and they can only associate learning with whatever part of the situation they are aware of.

You can also set how strong you want the suggestion to be. For example creatures have a strong instinct that eating food reduces hunger for fat, and a weaker one that pushing dispensers will reduce it... If they are hungry and see food and a dispenser, they will go for the food because the food instinct is strongest... if only the dispenser is around, but no food is in sight, then they will be more likely to try the dispenser... then when the food comes out, the stronger instinct will kick in and it will go eat the food.

Stimuli are a bit more limited in the situations they cover, though there is a pretty big list of possible stimuli... This means you may not be able to create stimuli that directly reinforce some instincts you might be able to make. The stimuli might cover a more general situation, including something you don't necessarily want to reinforce, but this might be okay... As long as you aren't actually punishing it for following its instinct, it should still keep following it.

A good example is the dispensers again... norns have an instinct that pushing dispensers reduce hunger for fat... however, there is no stimuli for pushing dispensers, only a more general one for activating machines.. and you don't want every machine to reduce hunger, or for them to learn pushing just any old machine reduces it, so you can't actually put in a stimulus for it directly. The instinct is really there as a backup suggestion for them on how to get food... They get the stimulus which gives them the hunger reduction after following their stronger instinct to go push the food that comes out. But to make up for a little less versatility, stimuli can affect up to 4 drives at once, which can be very helpful.

Overall, instincts allow for more programmed behavior, while stimuli kind of let them learn more naturally from experience (but they are not very likely to try anything they have no instinct for on their own, especially if they have other instincts that match the situation... Stronger instincts always will take precedence). CFE creatures are better at learning from stimuli, and don't rely as much on instinct as other creatures. They are better at discarding instincts that aren't helpful, and reinforcing those that are, by what they learn.

In your case, which you use and how you set the sliders really depends on how much you want them to crave plants right off, how much they should prefer them over other foods at first, and how much nutrition (and learning) you want them to get when pushing them. If I were you, I'd look at the existing instincts and stimuli for other foods to get an idea of where you should set the sliders. You can always just replace those other instincts, make them dormant, or make them weaker if you really just want your norns to strongly prefer plants. A combination of an instinct and stimulus are definitely the most effective for learning.