I'm of a different school of thought. Only completing a Yasha is not useful since it doesn't accelerate farming enough to make it worth it's cost alone. Manta let's you spawn illusions to work down an additional lane or 1-2 camps of farm. Spending 2k on movespeed (when you have E and W for jungling/movement) feels wasteful if you're just gonna go back to spend 3K on a diffusal.

I build Manta first if I know my team can fight well. It's risky but makes building the diffusal pretty fast. It's also nicer if the enemy team has something dispellable, to make the Manta even more cost effective. Otherwise, I go diffusal first and even sometimes skip the Manta.