Both are BL, Japanese, fairly short series, and really great romances. Love on the line was way more raw and emotional whereas His was more soft and cute, but I really enjoyed them both and I feel like if you liked one you would definitely like the other. The beginning of Love on the line has the sort of coming of age and first love vibe that His has throughout.

Thank god It's Friday is set in the same school with the same music club as Love Sick. The connection is shown in episode one. Also, both are set up as BL. TGIF has more characters and a set up for multiple couples it sees. Similar vibe between the two of them so far as well. The relation is slight however so I'm sure you could get away with watching TGIF without watching Love Sick. However, I would recommend watching Love Sick first because it's really good and having seen it you can more fully appreciate the setting of TGIF.

Similar concept of tutor and student. Beyond that they are pretty different. You had me at hello is longer yet much less developed. Both are cute and short.