Well, with the E6K you need to do some clever conversions to utilize the wideband. I could only ever get it to display voltages in a log, then you have to convert them to AFR outside of the haltech software. I do know there's a wierd version of the tuning software that would allow you to display AFR but I could never get it to work - although that may have been because I was using an X.
The narrowband is only useful for closed loop with the K and X, but if you get a wideband with a simulated 0-1v output you can use that to power the CL in the haltech.
Really depends on what you intend the ECU to do. I never had ANY issues tuning perfect cruise without closed loop FWIW.
Also, I have an AEM tru boost sitting around. Works excellent as far as boost controllers go. For what we're doing here that is. Its got a great alarm/overboost feature (tested first hand at 25psi
), holds a hi and lo setting, peak recall etc. Only issue I had with it is the display is digital (since its a boost gauge as well) and i found it tough to glance, read a number, and think of whether or not its where it should be etc. In that regard I prefer a needle type gauge but its just a personal preference.