Fortunately, the half-fiend/half-celestial difference is slightly greater than the fiendish/celestial one. Slightly.
Half-fiends obviously have other SLA's than their celestial equivalents, which are about equal in power. At worst, they're a bit weaker at higher levels.
Half-fiends fly speeds are way worse. Maneuverability is down by one step (which removes the ability to hover, move back, or reverse), and the speed itself is halved.
Half-fiends do get a better immunity (poison rather than disease), better resistances, and some natural weapons, but have lower ability scores overall.
Even moreso than with half-celestials, I find myself tempted to put half-fiends at +2 and see how the community responds. Still, I'll do the safe thing and rate them as +3's for now.