Just did the maths... here we go;
Sentry Cloak:
4 agility (4 attack power, 0,4% crit, 0,8% dodge and 8 armor)
5 stamina (50 health)
Spidersilk Drape:
4 stamina (40 health)
4 hit rating (1,89% chance to hit)
SP can be ignored, no function for a rogue.
What this does for your damage;
0,4% crit effectively increases your damage output by 0,4%, 1,89% chance to hit works the same and thus increases your damage output by 1,89%. 1,89% minus 0,4% equals 1,49% more damage output from the Spidersilk Drape compared to the Sentry cloak, the AP from Sentry Cloak not calculated with it. 4 AP will be no more than 1 weapon damage, which is for every decent rogue no more than 0,8% increased output. 1,49% minus 0,8% equals 0,69% increased damage output from the Spidersilk Drape compared to the Sentry Cloak.
What this does for your defense;
You lose 10 health, which is not much but always something you have to put into your balance. Then there's the 0,8% dodge which is imo the most over appreciated stat ever in the history of rogues; you only really benefit from it against other rogues and warriors. The 8 armor can be ignored, it adds less than 0,1% damage reduction for every type of rogue.
What this means;
Spidersilk Drape is definetely better for offense, and Sentry Cloak is only slightly better in defensive stats; yours to pick but I say
Spidersilk Drape. =]
~Rayu