I like the Time Serts, but the Kits are expensive. They have the normal thickness and double thickness if the hole is really messed up.
I think the Key Serts have both thicknesses. main difference is how the inserts lock. Key Sert you lock be driving pins down the side of the threads which may bot be good if the hole needs to seal, like going into a water jacket.
The time sert uses a counterbore bit so the insert has a lip on the top that keeps the insert from going down into the hole when using the thread forming tool. On the insert, the bottom few threads are fully formed, so when the thread forming tool is ran into the insert, it forms the last few threads while locking the insert into place. These work really good if needing to seal the hole. Because they are held in place by forming the last few threads, you do need to make sure to get the correct length insert.