hmm, not too up on the charmed world but the sisters, as hot as they were, seemed pretty weak to me. Willow could have eaten them for lunch.
so their 'demons' can't be that powerful can they?
I'd say Spike would win more often than Angel. Spike's probably the smarter of the two, he doesn't blab as much as Angel nor does he hold back as much. he relishes combat.
if it were Angelus, I think it'd be even or in Angelus' favour:
Spike v Belthazor: 6-4
Angel v Belthazor: 5-5
Angelus v Belthazor: 7-3