The slant head has been marketed as a "performance" part... but details are somewhat hard to come by. The angled plug would most likely be to facilitate
spark plug indexing so, with that in mind it would prolly be "better" to have the plug on the intake side (exposing max amount of unobstructed spark/flame front to fuel/air) ... but there's some controversy about the effectiveness of indexing *shrug* I do know that in the quest for maximum performance it's more about attentiveness to the smallest of details and
all the details than just getting "performance" parts and bolting them on. There may not be a noticeable gain from any one tweak - but add them all together and that's the winner, the devil is in the details lol
What interests me as I've not had a slant head and can't seem to find a pic of the combustion side - is there actually some semblance of a squish band*? This I would think FAR more important than indexing and sadly the stock heads have not even the faintest thought of one, it's just a simple dome... pics would be ossum ifn ya can
*squish band: a narrow section of a combustion chamber in which the fuel charge is more compressed by the piston than in the rest of the chamber; designed to help direct the flow of the fresh charge and to improve scavenging
edit: BTW for those whom may not know & controversy aside - to properly index a spark plug you'd need to remove the head & shim the plug so the side electrode (ground) is away from the center of the combustion chamber exposing the maximum amount of the center electrode & spark to the intake side.