Thie thieves guild has a place for everyone, no matter your disposition or inclination. Once you reach level 50 you will have to make a choice between the five "specs" as thay are commonly referred. There is information scattered about these pages on the various specialisations. This page is more of an overview of the five from which you can choose.
Your atypical thief. Everyone starts out as a prowler, many move on to other occupations. The ones who stay enjoy a well rounded group of thieves. The only drawback is a lack of fighting primary as thrown is not very useful in a fight. The covert skills mitigate this a little as it is hard to hit things that are not there.
Easily the largest group within the guild. cutpurses are masters of theft, using covert skills and sleight-of-hand to procure their loot. PIerce weapons, daggers and the like are the preferred choice of these men and women.
Patience, a calm disposition, and a penchant for solving puzzles are the things that make a good safecracker. They are masters of getting into, and out of the most secret of places, laughing at the traps put in their way. Not the best when it comes to fighting or "traditional" theft. These folks deserve respect.
Smugglers do not have a stealing primary, but they do not need it! A good smuggler knows the value of all that lovely loot, and how to get the best price for it from the fences. Smugglers prefer a sword over daggers.
The heavy hitters of the guild. The extra combat abilities come through a sacrifice in some of the covert skills. This group is for those of us who belive violence IS the answer (and pie).
As noted above this is just a simple overview to get you thinking about the choices you have. I encourage you to talk to various people (in private) and see what they have to say. Asking on the talker is fine too, just be prepared for the eternal debate over who is best (done mostly in a light-hearted manner).
In reality, there is no "better choice" out of the five. Pick the one that interest you, or suits your style and go with it. Once you advance to a suitably high level things start to even out amongst us all. If you are having fun with it, you made the right choice.
http://discworld.starturtle.net/lpc/playing/guild_ob.c?guild=thief To see what skills are primaries for each spec.