Eh, besides finding Bruce pretty overrated in general, from what footage I've seen of Maiden live recently he's not going as strong as everyone says he is. For instance the Number Of The Beast scream seems way lower than it used to be, whatever all the Maiden fanboys yelling 'he can still do the scream!' might think; it's originally an A5, a note that James has hit live fairly recently even though he's apparently in worse shape. I call shenanigans - you can tell James has aged, for sure, but for a singer whose vocal chords have been through so much hell I'd say he's one of the better-aged ones out there.
As for 'technical' superiority, let's actually examine the only real measuring stick we have, shall we? You can't exactly measure vocal power, but range is definitely discrete data:
James: C#2-A5 (he hits his lowest in In Too Deep from Elements Of Persuasion and his highest, funnily enough, covering Number Of The Beast, though he's also hit it during performances of LITS before)
Bruce: B1-F5 (this is going by research someone else has done, but it's good research
https://www.youtube.com/watch?v=uIOqU-Y3TEQ)
Keep in mind this is full voice notes only - Bruce extends his range by nearly half an octave through the use of falsetto. James doesn't have much vocal fry or falsetto stuff, so pretty much all his recorded notes are full voice. I have my own conclusions (he does a fry part while singing to a fan on one of Jordan's vokle chats, and I think he hit somewhere around A1, and I think - I THINK - he hits a falsetto B5 in 'Coming Home', but my ears might be an octave out), but for the purposes of this discussion fry and falsetto are unimportant as they're basically artificial extensions of range rather than part of a singer's actual voice.
What we can see from this is that Bruce has 2 semitones over James in the low end, but James has 4 semitones over him in the high end, resulting in what is objectively a wider vocal range, if only slightly.
Anyways, as for my personal preference, I think James in his prime is unbeatable by anyone ever. He has the versatility, the strength, the whopping range, and a very distinctive, identifiable vocal timbre. Bruce I will respect as a pioneer, as one of heavy metal's first wailing, operatic frontman, but James has my favourite voice in the world and back then it was flawless.