Quoted from Intel.com HERE
Intel® Trusted Execution Technology
Intel® Trusted Execution Technology for safer computing, formerly code named LaGrande Technology, is a versatile set of hardware extensions to Intel® processors and chipsets that enhance the digital office platform with security capabilities such as measured launch and protected execution. Intel Trusted Execution Technology provides hardware-based mechanisms that help protect against software-based attacks and protects the confidentiality and integrity of data stored or created on the client PC. It does this by enabling an environment where applications can run within their own space, protected from all other software on the system. These capabilities provide the protection mechanisms, rooted in hardware, that are necessary to provide trust in the application's execution environment. In turn, this can help to protect vital data and processes from being compromised by malicious software running on the platform.
In a nutshell, it's supposed to help prevent viruses. I haven't heard much about it and I don't know how well it works really. The best way to always go is with a good anti-virus.
Why would the more expensive processors not have it?
The more expensive faster processors (9XX Series) don't have it because they are actually the older processors of the i7 bunch using the Socket 1366... X58 chipset. The i7 8XX Series are very new with a release just a couple to few months ago. They use the Socket 1156... P55, H55, and H57 chipset.
Would it be better to get one with the higher bus speed or one with the Trusted Execution Technology?
Depends. Are you looking for overall processing power? Or, more interested in dropping down a step and concentrating on security a little? Both series of processors are very nice and for the common user are more than enough to do what they need it to do.
Hard Drives play an important role in how speedy a system seems when it comes to the common tasks of installing programs, how fast programs open up, and/or transferring files to and from partitions or USB Drives. So that may be more of what you are looking for. Heavy duty processors like the i7 Series are great for video/photo editing and gaming. And things like 3d Rendering in CAD programs.
What will you be using the computer for?