I would recommend learning both legato and staccato techniques.
Both are very useful and I believe that there is no fixed rule on which one to use.
There are rules, nevertheless. Rules related to the correct way of what you are playing.
For example, traditional Airs songs should ideally be played in accordance with the vocals.
So even if you wanted to put a staccato and/or a legato, ideally you should not because you are replicating the vocals, so you should play the words.
Both legato and staccato techniques are tools that you can use to create a particular mood, rhythm, ‘phrasal’ statement or emphasis, etc.
There will be times that you will prefer using staccato in a particular sequence, other times you will prefer using legato and other times both.
The type/genre of the tune also provides clues on what technique would suit the tune, or your feeling.
I found that on medieval music staccato tends to be used more often (because it feels right).
On fast Irish tunes I tend to use less staccato (but depends on the tune and the mood).
On slow tunes, like Airs, I tend use even less or no staccato.
There are players that do not use staccato at all (the whistle replicating the bagpipes).
There players that use staccato intensively in any style of music (including fast Irish tunes).
I believe that the use of legato and staccato techniques also depend on your background and musical influences.
And all your musical experience in time will contribute to your own way/style of playing.
Regarding legato and ornamentation, with practice and dedication you will easily develop them.
What we call ornamentation can indeed be necessary to define the tune, so it may be not an ornamentation but part of the tune itself.
This means that what sometimes seems an ornamentation it is not (however the playing mechanics is the same).
The whistle is one of the simplest instruments to learn to play.
With a few lessons you will learn all what you need (just a few, I would say something like 15 things or so to learn), the rest is practice, dedication, perseverance.
It is important to listen as much as possible to the type of music you want to learn.
With time and dedication you will start to develop and understand what technique to use.
And, together with your own style of playing, you will apply the techniques suitable to the mood/feeling you intend to deliver.
It is important to play regularly. It does not need to be for long periods of time but it needs to be regular.
It is better playing every day, like 5 minutes per day, than playing long hours on just one day per week.
Doing in that way is better to your brain. It helps reinforcing what you are learning and develops muscle-memory.
I also recommend starting learning and playing tunes that you like, because if helps with your motivation.
Do not be afraid to learn to play what seems fast or complicated tunes (sometimes it is the ornamentations that do the trick/illusion).
If you slow down fast or difficult tunes and play them slowly regularly and by parts/chunks/frases you will see that they can all be learned.
There is something important to refer regarding the staccato.
Staccato separates the notes. However you will find that you might not need to separate the notes strongly.
You can use a very soft staccato to get clear notes ins such a way that even when playing fast the notes flow close to a legato.
It is hard to explain using words but a soft staccato can be achieved using the tongue like saying a soft and rapid letter ‘R’ with your tongue touching the palate.
Another soft staccato is achieved with a quick touch of your tongue on the top of your palate like saying a very soft and quick ‘D’.
Whistles are amazing instruments.
It is overwhelming the types and amount of music you can play with such a simple instrument.