Guard or shield pulse?

Apples and oranges man. If you want to prevent armors from shooting a one-turn Ignis or Eclipse to your face, always go with Guard.

Shield Pulse does not affect the enemy special in any way. What it does is, if you have a defensive special like Miracle or Aegis, you get two free charges at the start of the battle, much like Quickened Pulse. Also, if your defensive special triggers, the damage formula will add a -5 damage at the end of the calculation. For example, my Corrin takes a Naga tome to the face that does 50 damage, but I have Aegis and Shield Pulse, and Infantry Pulse, so I have it charged from the get go.

The game does this:
50/2-5: 20 damage.
and below 10 damage, you take nothing
=<10/2-5:0 damage

It can be really useful to take on units you would normally die against, but it costs your own offensive special, so it's only really useful on super-powerful tanks that don't need more damage, or dragons that due to the adaptive damage will usually kill from the base damage.

Here is a demonstration. I use Aegis and Deflect Magic (considering to swap it with Flashing Blade now) due to having Infantry Pulse support.
https://www.youtube.com/watch?v=fYebRkw_BP4

The problem with Specials like Aegis and Pavise is that they are too "passive", Seriously even with normal attacks it will take effect, sometimes there are situations that it will take effect on a normal attack then the enemy attacks again but this time it has special proc and since aegis/pavise was used before when it should have been used when the special was proc.
Unless it's miracle since it can do some clutch moments.

Depends on what you prefer, do you want to stop the enemy from coolingdown the special in the first place or reduce the damage taken with passive specials.

IMO go with Guard cause it's more consistent and if you built the unit properly the unit will take the least amount damage or 0 so the unit will be over 80% of health.