I'm going to approach this from a different angle. I'm going to look at my personal experience when watching the show and how that affected my opinion of Spike and Angel respectively.
Personally, I never saw it as an 'either/or' issue. Although I've come to favour Angel, that has little to do with my feelings for Spike. I mean, I prefer Wesley to Andrew, but my liking Wesley as a character has little to do with how I feel about Andrew.
I actually like Spike and I personally don't believe he, himself, took anything away from the televised shows in general. In fact, I'd go as far to say he added quite a bit. So, I reiterate: While I prefer Angel to Spike, I'm not 'choosing' him over Spike.
The reason for position is that contrary to what some have said, I don't believe the narrative placed Angel and Spike at odds with one another. In season 1 of BTVS, Spike wasn't present. In Season 2, Angel had a soul for the first part of the season and Spike was one of the main antagonists and an entertaining one at that!! I appreciated both characters in completely different ways. When Angelus emerged, I couldn't really say I was rooting for him over Spike, because, well they are both villains. If anything I tended to sympathise with Spike, but in any respect, I still enjoyed Angelus as a villain, so again, no real preference there.
In Season 3, Spike was barely there and was a villain, while Angel was on the hero's path. Even in Season 7 of BTVS, Spike and Angel were treading different paths or at least there were at different stages. Spike had just won a soul, while Angel had spent years already as a champion and a leader.
It was only in Ats Season 5 where the competition on screen was pronounced, but even then, in the later part of the season, Angel and Spike were cooperating and the Bangel vs Spuffy issue was mostly played for laughs.
Yes, you could argue their competition in the 'Verse lasted over a century, but even then, for most part, they were not in competition with one another.
I actaully found a lot of the competition is noted online on the forums. The common issues I've seen discussed are:
1) Shipping
2) Moral superiority
This can include, the fact that Angel has been fighting for good for longer, or that Spike won a soul etc...
3) Who deserves the Shanshu
None of those issues mean anything to me because
1)I don't care with whom Buffy ends up.
2) I don't see heroism as some sort of scoreboard, such as "X has saved the world more" etc. Both of them are on individual journeys and their heroism can be appreciated independently of each other.
3) The Shanshu is not necessarily a determining factor in regards to which character I prefer.
That's all for now, but as always, this is just in my humble opinion.