Nowadays they seem mostly interchangeable but Marvel appears to be doing slightly better.

Yes indeed.  When I started reading comics in the late 60s, DC's stories were plot-driven, and the plots were often clever, complicated, and gimmicky. Marvel's stories were action-driven and often not overly complex, and it was not unusual to have several pages of characters simply slugging it out in a typical issue.  DC's characters had more self confidence and were less plagued by neuroses, and their private lives tended to be uncomplicated.  Marvel characters were neurotic and their personal lives were like a soap opera.  And DC's were clearly pitched at younger readers.  Clear differences.

This started to change in the early 70s with the influx of new writers at DC who were Marvel fans.  It changed even moreso in the mid-70s as established Marvel writers like Conway and Englehart began emigrating to DC, along with the ascendancy of fan-turned-pro Paul Levitz to editorial power.  And you're right... by the time of Crisis, any meaningful distinction in storytelling approach between Marvel and DC was completely gone, as DC wholly embraced the Marvel style.

It also does puzzle me that there is any kind of brand loyalty post-1985.  I guess in the post-Crisis world, brand loyalty is determined simply by a preference for one set of characters over the other, rather than a preference for a certain type of storytelling approach. 

Even when DC seemed to "Marvelize" in the early 90's, Marvel itself was turning into Image!  (All those muscles!  Muscles upon muscles!  And women with broken backs!)  Don't forget--for about five years before Image formed, all those artists were doing their thing at Marvel.

Right now, DC is totally pursuing that "classic" Image look--the artwork (especially the inking), the coloring, the paper, the sort of "overblown-ness" of it all--reminds me of late 90's/early 00's Image.  Meanwhile, Marvel seems to be pursuing that "Indy" look and feel--decompressed and talky storytelling, way too much art with "coloring book" lines waiting for the colorist to do all the work, etc.

I suppose for a short time just after CRISIS in the late 80's, both companies were doing their best impression of Marvel, but usually they seem very different to me.

I only ventured to DC to follow creators and even then the characters didn't hold my attention like Marvels.