Speed is Shulk's best bet at acting as a rushdown character with approaching Nairs, grabs, and jabs/tilts for light damage. That art is meant to help Shulk dictate the flow of the match as, while his jump height and offense is weakened (damage, not knockback), his speed rivals Sonic's. It's also generally effective as unlike Sonic, Shulk's attacks cover very nice range thanks to the Monado's beam. Speed's not as situational as Shield and can be used pretty much whenever you get a good rhythm going, though I recommend at the start of the match for some easy combos or when the opponent comes in with a new stock.
Shield on the other hand is if for playing super defensively and stock turtling. This is for whenever you're at a high % and not standing near a ledge where someone can easily throw/launch you away from the stage. Again Nairs and tilts come into play here thanks to their range and overall effectiveness. Oh and your shield health is increased during the Shield art, enough to not break after a Bowser Bomb or Marth's Shield Breaker.
