I'm fine with either way, as long as it's established before it becomes important.
As it's written:
"the character makes an Opposed Test against the target’s Perception + Intuition" (SR4A 136), it seems to imply that the person doing the sneaking is the one engaging in an action the "target" defends against with perception... In which case 2 Perception beats 2 Infiltration.
But it's easy enough to interpret the other way around.
Not like it changes Silk's reaction
Imo, on the field, successfully sneaking past guards should always be done trying to get as many visibility modifiers as possible from bad light, distraction, distance, camouflage/chameleon, blending in, etc.
