Both are very good boons but I personally prefer a speedy Libra. He's a little more flexible in his role this way, as he can choose to tank, go mixed phase, or even sweep vs. being confined to a strictly tank role. Even without defensive investment he has good defenses to work with.
I have a +10 Libra, and I decided for +Spd/-HP as his final and "perfect IVs". He can honestly take any route he wishes, and IVs point him to one direction or the othe.
On one hand, +Def/-Spd kind of takes his Spd off the table. This means you can focus on more defensive stats, like extra Def and/or Res investment and make him a full-on Enemy Phase tank with stuff like Double Close Def, QR and Hack O' Lantern as a weapon (assuming you don't mind this investment). His Spd is also not as bad that he'd get doubled by some of the Dragons or something.
I like +Spd because it allows him to be more mixed phase. And when he's that fast with stuff like Handbell/Santa Bag, he doesn't even need stuff like QR since he'll double pretty much anything that isn't Ayra/Karla/Cirque du Soleil. In this case, Fort. Def/Res might be a really good chase for mixed phases.
My Libra:
Pokemon GO
Fire Emblem Heroes
Fate Grand Order
Dragonball Legends
Dragalia Lost
IV Calc
Heroes