I would say "usually" the answer is boots. But it's situational. My arena team, for example, consists of an AOE provoke - which means I don't really need my nuker to be that fast. I'd rather have them be slower and just oneshot the entire enemy team instead.

I would say, though, that anything below 150SPD is extremely situational. Even in my aforementioned case, my nukers are still around 170-180 SPD.

I would say build you fastest turnmeter booster champ as fast as possilbe and then speed tune.

If you 

a) Can use att% boots and still be speed tuned go for it

b) Have a reliable 100% CC champ you can go out of speed tune with att% boots and wait for enemy reaction sets to wear off.

If none of the above go for speed boots.

Thank you I will follow your advice.