For me, the two operative ideas are "design philosophy" and "best of breed."
I think that at the higher end of the spectrum most equipment will sound good. If I just avoid the obvious incompatible matches, I should be aurally happy with the result. Beyond that, I have always been attracted to the design philosophy behind a product. For example, the Well Tempered Turntable and Arm were tremendously exciting for me to read about. Bill Firebaugh came up with innovative, elegant engineering solutions for his table and arm that really appealed to me conceptually. (I never bought one, however.)
It is relatively hard to compare two arms. For direct comparisons, I would need the same cartridge, table, electronics, speakers, and room. It's hard enough just to find a dealer that carries both brands that I would want to compare. I have a similar issue with tables. The best view that I have been able to get is when I upgrade to a new system, and I can compare the new one's sound against my memory of the old one. Of course, it's always better.
To compare three high end turntables or arms, I wind up studying the engineering design concepts and trying to glean insights from reading reviews or talking with others who have spent time with them. At the showroom, they all sound great.
When I buy a high end arm at high end prices like the Graham, I feel like I'm buying a piece of form-follows-function engineering art. Not only do I expect it to sound good, but I need to appreciate its engineering aesthetics.
I also like to think I can pick and choose the components I like the best, for a "best of breed," at least "best" as determined by my own preferences. I happen to like the HR-X and the Graham arm -- why shouldn't I have both?
VPI and others are working to convert the separates market into a systems market, bundling arms and tables, arms and cartridges, and some even bundling all three. This has business advantages in that the same manufacturer captures a higher proportion of a playback system purchase. Perhaps more importantly, we know the set up will be compatible, and the manufacturer will make sure that potential synergies are realized.
For me, it becomes a little bit of an ego/freedom-of-choice thing, that I can pick a satisfying combo from an open field. In line with this, I have several systems but only one has a receiver (MD 208) versus separate amp and tuner. So, in the end, do I have my own opinion? or, should I just default and defer to the manufacturer's desire for higher market share and revenues?
Very philosophical.