I think it's easier to get better beer from a stainless conical - it doesn't magically make your beer better if bad habits abound, but if you leverage its advantages like the following, the sum of the small improvements in brewing make better beer more likely more consistently:
- stainless steel much less prone to harboring remnants from previous batches/uncleanliness - this is particularly true after several uses as the metal won't develop imperfections/scratches/etc. as rapidly that make that an increasing possibility
- because of ability to dump trub, you can use one vessel for entire fermentation and pre-packaging conditioning which means less exposure to outside air or other contaminants (that is, if you are introducing CO2 to replace air volume lost when trub leaves the dump valve) from eliminating racking to secondary and also allowing direct transfer to keg in closed fashion (if you keg your finished beer)
- racking arm/valve allow sampling to monitor gravity, clarity, flavor, aroma, pH, etc. to better monitor fermentation
- it could just be me, but even when I've done CIP on my plastic buckets, I still smell a distinct aroma of yeast and/or hops (degrees on both depend on what was just in there) for a while afterward. I don't have that with my stainless conical, and it begs the question how much that contributes to the next beer
- some say 2nd and 3rd generations of yeast can be more viable/virile than the initial pitch when harvested properly (i.e. from the middle third of the cone at optimal times), possibly because of selecting yeast with best floc profile and other desirable traits - conical makes it possible to more deliberately harvest the particular layer of yeast to try and achieve this
- does prep you to work on pro equipment if you find you want to take the hobby further...possibly professional. I'm not sure if this is a thing, but I'm guessing people are also more inclined to buy a used stainless fermenter than a plastic one because of sanitation and durability concerns (edit - this "pro" doesn't relate to making better beer, necessarily)
Having said all that, I do still brew in my plastic buckets fairly often since they fit in my temp control keezer, and I've brewed pretty decent beers in them.