Here is a brief write up on pre-turbo and post-turbo. And I always recommend ISSPRO gauges, but I'm a bit biased
Pre-turbo or Post-turbo thermocouple installation?
Clearly the manifold (pre-turbo) installation is better in almost all circumstances. Many people from the "old days" worry about thermocouple tips breaking off and chewing up the turbo. This fear is a relic from the days of exposed-junction thermocouples (the type still used in laboratories). For 40+ years the automotive standard has been to encase the thermocouple junction in a welded sheath of high-temperature stainless steel. More recently they have also been made out of Inconel stainless, which is rated for continuous use at 2000° F. Here at ISSPRO we have not had a single report of a sheathed-design thermocouple breaking and damaging a turbo, with over 40 years of history.
Measuring pre-turbo tells you more about the temperaures your pistons are seeing, as well as the worst case temperature of your turbo. The temperature difference between pre- and post-turbo can vary anywhere from +500° F (pre-turbo much higher when under heavy load and temps rising quickly) to -100° F (post-turbo hotter immediately after starting downhill after a hard pull uphill).
The only time we see post-turbo measurements as preferable is when monitoring the turbo temperature during shutdown. As the turbo cools off, it is being cooled by the exhaust gases at no-load (which are now cooler than the turbo). These gases get heated up as they cool off the turbo, so you actually see a warmer temp post-turbo. However, the difference between pre- and post-turbo temperatures is minimal by the time the turbo has cooled to around 300° F (which is where most people shut down). In other words, post turbo lets you see the cooldown progress better, but it is nearly identical to the pre-turbo reading by the time you reach the shutdown temperature.