Alright, young Shaper, let's talk decoders.
If you're looking for a main breaker, there's really four main options: ZU.13 Key Master, Gordian Blade, Torch, and the new Study Guide. There's also Refractor, Yog.0, and Cyber-Cypher but those all need specific support to be useful, so let's just look at the three standard decoders for now.
If you're debating between ZU, Gordian Blade, and Torch, the real question is, how big is the ICE you plan to break? If you expect to mostly be staring down Quandary and Enigma, then Zu will do the job at nearly same cost as the more expensive breakers and for a helluva lot less up front. On the other hand, if you plan to be hitting the same Lotus Field over and over, you'll be glad you paid extra for the big grandaddy Torch.
This is more important the more times you are going to be running. If you're breaking a Lotus Field, it will cost you 4 per run using ZU, and only 1 per run using Torch. But you paid an extra 8 to play the Torch in the first place, so you need to make at least 3 runs for the initial investment in the Torch to pay out.
The problem is that you have to decide which breaker you want to build your rig around before you ever see your first piece of ICE. Opting for ZU means you can get set up and run faster, but means that big code gates like Tollbooth or Wormhole basically shut you down. Torch gives you the power to burn through any ICE...eventually, and that means the corp has lots of time to set their own board up.
But then there's Study Guide. Where the three other decoders are essentially small/medium/large, Study Guide is one-size-fits-all. If you're facing down weak code gates, then you only need to charge it up a little bit to have a breaker that is strong enough to handle them. On the other hand, if you're looking at monster ICE, you can make SG an even bigger monster. You never get caught in the embarrassing situation of having to break a Quandary with a Torch, or a Checkpoint with a ZU.13 Key Master.
There is a price to this flexibility, though. Yes, Study Guide can be a weak breaker, but it costs 2 more to install than Zu. Yes, Study Guide can be a strong breaker, but it costs a total of 11 to install and get it to four strength, two higher than the 9 it would cost to install Torch. And even though you can break a Wormhole for 1 after boosting it to strength, it's going to cost you a staggering 17 to get there first, with the saving grace that you don't need to do it all at once. Study Guide is jack of all trades, and a master at none.
Kit always deserves special mention when it comes to decoders, and in this case I think she's really happy. Kit's greatest strength is early-game aggression: being able to hit any single-iced server is terrifying for the corp until they manage to double-ice their centrals and scoring remote. But she also needs an efficient code gate breaker, to deal with high-strength sentries and barriers turned into code gates from her ability. Study Guide is the only decoder that fills both the early rush need and the later power-breaker need.