HTL is the original Blu-ray format, LTH was developed later to reduce costs. The reason for the low cost of LTH is that its production process is similar to that of DVDs.
Which is Better?
Generally speaking, HTL discs are considered better. They are more resistant to UV light and moisture. However, it is possible that a specific LTH model is better than a specific HTL model. For example, an LTH disc manufactured by Taiyo Yuden might be better than an HTL disc manufactured by Ritek or CMC.
How to check whether a disc is HTL or LTH before buying?
The general rule is that if the disc package does not say whether the disc is HTL or LTH, it is most likely HTL. This might sound counter-intuitive since HTL is, generally speaking, the better and the more expensive option. LTH discs are not compatible with some very old Blu-ray players, this is why LTH discs are usually labeled as such, as a warning.
Check whether a disc is HTL or LTH using Imgburn
From the Ez-Mode picker choose any option which involves reading from or writing to a disc. There will be a device pane on the right which shows the information we need. The mode which most people will be familiar with will be the write files/folders to disc mode. In this mode, the right-pane has multiple tabs and the device pane is one of the tabs.