Lifestealing (same with Crusader and others) are based on "proc per minute" so your comments about putting a faster or slower proc-based-enchanted-weapon in the offhand are mistaken -- it doesn't matter.
What does matter, however, is that your instant attacks (e.g. Sinister Strike) are made with the mainhand. So if you want a proc-based enchant, put it on the mainhand -- the "proc per minute" calculation is based only on autoattack so if you throw instants into the mix you're getting bonus chances to proc.
Because of the lack of instants, it's recommended to put a non-proc-based enchant on the offhand, +15 agi is a good choice for Rogues and Hunters.
Also, for your instant attacks, you want a weapon with the highest average damage-per-hit in the mainhand. For a given DPS that means a slower weapon in the mainhand, but don't just go by the mantra "slower weapon in mainhand is better", because that's an oversimplification. In a head-to-head comparison, even if you add in the AP benefits of dual-wielding them, Cruel Barbs simply don't match the average damage-per-hit of Shadowfang, and the shadowbolt proc is icing on the cake. So Shadowfang should be the first choice for mainhand.
Of course, all of the above is somewhat of a simplification. If you start talking about different weapons with different stats, such as strength, agility, stamina, then it gets into complicated formulas involving dodge%, crit%, regen rates and so forth. Since I don't have enough space for all of that, let's just say, for a 19 twink Rogue:
Disclaimer: my twink Rogue is 29 not 19. But based on the same calculations I'll be applying the Lifestealing-mainhand-+15agi-offhand enchants to his dual Torturing Pokers Real Soon Now. Already have Fiery on his Ambushing weapon (Claw of the Shadowmancer) so I'm set there.
What does matter, however, is that your instant attacks (e.g. Sinister Strike) are made with the mainhand. So if you want a proc-based enchant, put it on the mainhand -- the "proc per minute" calculation is based only on autoattack so if you throw instants into the mix you're getting bonus chances to proc.
Because of the lack of instants, it's recommended to put a non-proc-based enchant on the offhand, +15 agi is a good choice for Rogues and Hunters.
Also, for your instant attacks, you want a weapon with the highest average damage-per-hit in the mainhand. For a given DPS that means a slower weapon in the mainhand, but don't just go by the mantra "slower weapon in mainhand is better", because that's an oversimplification. In a head-to-head comparison, even if you add in the AP benefits of dual-wielding them, Cruel Barbs simply don't match the average damage-per-hit of Shadowfang, and the shadowbolt proc is icing on the cake. So Shadowfang should be the first choice for mainhand.
Of course, all of the above is somewhat of a simplification. If you start talking about different weapons with different stats, such as strength, agility, stamina, then it gets into complicated formulas involving dodge%, crit%, regen rates and so forth. Since I don't have enough space for all of that, let's just say, for a 19 twink Rogue:
- Shadowfang mainhand with Lifestealing (it procs more than Crusader = better proc-based enchant overall)
- Cruel Barb offhand with +15 agi
- If you're really into maximizing your Ambush/Backstab damage, get an Assassin's Blade and do weapon-switching, put Fiery Weapon on it (no need for the healing benefits of Lifestealing or Crusader when you're at full health, right?); +15 agi might be a reasonable choice also, if your crit% is not high enough for your liking
Disclaimer: my twink Rogue is 29 not 19. But based on the same calculations I'll be applying the Lifestealing-mainhand-+15agi-offhand enchants to his dual Torturing Pokers Real Soon Now. Already have Fiery on his Ambushing weapon (Claw of the Shadowmancer) so I'm set there.