Thieves advance themselves with better tools and items to make up for what they lack from assassins. There aren't always assassins that stay in the guild. Those who leave are usually subject to execution, but some escape, and even less are allowed to leave (retirees, or top mention assassins). Like I said before, assassins follow a code, whether they like it or not, in order to join the guild and learn better abilities and train.
Assassins rarely have to take jobs since most of their quests are high profile, which means a lot of money, and a LOT of time to hone their skills.
Actual fully-fledged or even master thieves are great in battle in the technical sense, but the trust level is so low, that 1/2 usually end up with a slit throat after a big quest.
This is the end of the explanations for this. This world is made by me. It's not based on any other.