This decision gets made on looks and regretted on maintenance, so it is worth separating the two.
What you are actually choosing between
Natural stone is quarried: granite, marble, quartzite, soapstone. Every slab is different, and what you see in the yard is what goes in your kitchen. Engineered slabs are ground stone bound with resin and pigment, manufactured to a consistent pattern.
The maintenance difference
Most natural stone is porous and wants sealing, typically once a year, sometimes less. Marble in particular etches: an acid, meaning lemon juice, wine, or vinegar, will dull the polish where it sits. This is chemistry, not damage, and it cannot be prevented by sealing.
Engineered slabs do not need sealing and do not etch. They do have a heat limit, and a pan straight off the burner can mark the resin permanently. Natural stone does not care about heat.
So the honest summary: engineered material is more forgiving day to day, natural stone is more forgiving of the one careless moment with a hot pan.
The look
Engineered slabs are consistent, which is exactly what some people want and exactly what others dislike. If you want the counter to look identical from end to end, and identical to the sample, choose engineered.
Natural stone has movement that cannot be reproduced. The cost of that is that you should see the actual slab before it is cut, and you should be there when the fabricator lays out the cuts, because where the veining lands on your island is a decision somebody is making whether you are present or not.
Where the marble question usually lands
Marble in a working kitchen will develop a patina: etch marks, dulled spots, softened edges. Some people find that this is the appeal and some find it maddening. It is worth being honest with yourself about which you are before you commit, because it starts within the first month.
If you want to see the difference in person, that is the conversation to have standing in front of the material rather than looking at photographs.
