I find that 9 times out of 10 I take red smite because honestly it feels like it outclasses blue. Red smite does more true damage to champs and makes you take less damage in return, the only downside is having to AA in order to get the damage off.
Hyrum Graff3 votesRed smite is good if you can reliably get in aa range of your target. Blue smite is good if the rest of your kit is unreliable, since the slow helps you land stuff.
Certainly if you're able to land everything without blue, red is better, but reliability can be really good.
Dominick Destine1 votesRed Smite is good for duelists. 
Blue smite is better for tanks and assassins 
EndlessSorcerer1 votesHonestly, I find Skirmisher's Sabre to be the better option for many tanks.
I'm already loaded with CC, so I don't particularly need another slow. I'd prefer Challenging Smite to help me deal with invading counter-junglers since I'm not a particularly good duelist.
Dominick Destine1 votesHonestly, I find Skirmisher's Sabre to be the better option for many tanks.
I have found that to be a bad choice.
I'm already loaded with CC
That's correct, but tanks tend to be slow compared to nimble fighters/assassins. The ability to approach/initiate is a key factor in tank gameplay.
Åeolus1 votesI mean blue smite has a slow as well.
