Difference between LTO half-height and full-height drives?

Half height used to have limited connection options, however now support almost everything full heights do. Except maybe ESCON, but if you're using that, you have bigger problems (and budgets).

The main difference between the two now is maximum speed. The tapes are the same and interchangeable, but a half height goes maybe 70% the speed as a full height. Check the vendor's published specs to be completely sure.

I run a tape duping company and in my experience, the full height drives are more robust, and more likely to 'go the distance' if you're operating them all day. I haven't had a chance to compare HH and FH LTO5 and 6 drives, but certainly if you disassembled earlier generations of LTO (and before that, DLT) and compared the drive mechanisms, the FH parts were stamped from thicker metal and had been finished to a higher standard.

If you're just backing up a tape or two every day, either drive will probably be swapped out and upsized before it fails; if you're using it all day every day, go for a full height.

Oracle explained this in his document:

What are the differences between full-height (FH) and halfheight
(HH) drives?
Full-height drives are twice the height (2U) of the half-height
drives (1U). In addition to the differences in the height of the
drives, half-height drives historically have had lower
specifications than full-height drives. For example, LTO 4
half-height drives have a native throughput of 80 MB/s while
the full-height drive’s native throughput is 120 MB/s. With the
LTO 5 and LTO 6 releases, this trend has changed; LTO 5
half-height and full-height drives both have a native
throughput of 140 MB/s, and LTO 6 half-height and fullheight
drives both have a native throughput of 160 MB/s.

http://www.oracle.com/us/products/servers-storage/storage/tape-storage/lto-tape-drives-overview-faq-1906830.pdf