Smeargle has got one of highest speed and lowest Atk/SpA from all pokes who can use thief/covet. Thanks to that he will always hit 1st and for sure wont kill, only thing is that you will have to run (so you wont get exp while farming) but teleport works fine as 100% runaway. Spore/thief/tele/XXX and you have poke that will almost never get hit, in 2 turns can get item and in 3 run away.
Thief pros:
- Faster to make than 100lvl (if you invest in vitamins) - cos we meet smeargles at lvl 20 in wild.
- When you make it its 100% free, no need additional costs
- Is faster to use when we meet item carrier (always 3 rounds)
Cons:
- Sometimes we can meet poke with higher level than our farm target and have hard time getting hit.
- Require or using vitamins (so get 100speed ev in no time), or spend some time on ratatas, pidgeys at r1 farming speed.
- We need more than one to do work (different lvls - different thieves)
- Fight takes 3 rounds no matter if carrier has item or not.
- Each time we run we use teleport - so we visit Pokecenter offen than trick build (using RUN can easily fail on same level and we lose round), so if we dont have one close, we lose time.
Trick pros:
- You can farm any item with only one.
- Overleveling make it less stressful to make and easier to use.
- When we dont meet carrier with item its take 2 rounds to run/kill.
- We run for free from anybody.
- With payday we can get extra money for pokeballs (increase income) - but it doesnt OHKO some pokemons even on 100, so need to be used wise.
Cons:
- Each time you get item you have to catch pokemon. Its can take way more time than thief. 3 rounds setting everything (4 if we switch) + rounds to catch, which can be greater than one.
- Pokeballs cost. Maybe not much, but they arent free, special when we want use better one to make runs faster.
So. If we want specialist, who we use only on one type of item - Thief is better, do it faster have no extra fee. But if we want all rounder, who can get all items in earth from pokes - Trick sound better, special in places faaar from cities/hard to access. For me in farming is all about time. So if i have close Center (like Cinnabar or I6) i would use Thief. If we hunt for items in Cerulean i would rather take Trick.
But hey - we can have both and change when we want to.