Sorry ItsaGaAs, not sure if you mis-typed something, but your information is slightly inaccurate.
1 Agility = 1 Attack Power, not 5 = 1 as stated.
It does equal about 0.4 (rounded) dps, but the Agility also provides some crit. Based on the linked Wowwiki table, at that level 5 Agility equals about 0.15% crit. Not sure if that equals the remaining 0.9 dps gap or not, but it's probably close.
At level 43 it probably doesn't make too much difference, but based on the 5 Agility on the Mithril Blunderbuss, I'd say it may be a better choice. Since both are bound to you, just equip them and see how it affects your tooltip dps. Probably the easiest thing to do.