Of course the experience is different on both ships. Mainstrean ships generally have a lot more activities, but that is not what you are paying for on a luxury ship.
Lux ships have fewer ways to spend money onboard; smaller spa, less shopping, few to no alternative dining spots with an extra charge. The food is better overall all the time.
The atmosphere on smaller ships is more convivial - you meet people faster, and the service is far more personalized. And you are miving with people of "means" - all of them well to do; well traveled, educated and sophisticated for the most part.
ON a mainstream cruise a top suite guest is still vying for space in the theater with 3000 other people, but the shows are generaly better. The bare are open kater and there are more people out at night - if you care about that.
Basically a mainstream ship gives you the option to economize on your cruise, you certainly can cruise for far less than $3365, but you miss a lot of the fun and sight seeing.
Also - on a lux ship you can certainly pay a lot more, but this is what I noticed; since a lot of the "outside cabin" costs are included in the price (drinks, tips, tours) there is less incentive to move up to the pricier suite. The cost goes up dramatically, but for the most part all you get is more space.
But some lux ships offer extra tours, business class air, etc to people in top tier suites, but they are paying for those perks.