Naga Siren and Diffusal?

I'm more of a rush radiance kind of player though I've been told Diffusal is extremely good on Naga and it'd be great to try it out.

Though I'm confused whether you get it after Radiance or do you rush diffusal?

Radiance is a 'rush it or don't get it at all' Item. Next I'd probably go Manta Style as a core.

Well yeah that's my usual Naga build (though I like to rush Boots of Travel after Radiance. Though say I decided to get Diffusal Blade, would it be better to just build Naga DPS on Naga?

I meant the manta after the Diffusal. So yeah just get damage on her.

Ah so, Radiance>Diffusal>Manta>BoT?

If you go for randiance build (and im not sure how a naga without radiance is) then you maximise your splitpush power and farming speed.
Diffusal is for when you actually want to fight, and is bad against BKB and good against heroes that rely on mana.
Generally on splitpush naga you go radiance>travels>manta/octarine core. If you get a diffusal its usually after that, fighting early isnt her strong suit apart from directly after you get radiance if you have a good teamfight lineup.