Players of any instrument should be comfortable with up to 3 ledger lines. Measure 2 should be perfectly readable.
For many instruments, writing the part in the correct octave without an 8va mark is preferable because playing in different octaves means different technique. For trombone, the middle line D is played in 4th position, but the D an octave higher is typically played in 1st. So if a player sees a middle line D, he might instinctually start to go for 4th position, and then have a little mental hiccup when he notices the 8va mark. It's not a huge deal, but it can impact sight reading. This is more important for woodwinds.
You can also change clef. Trombonists, cellists, and bassoonists should all be adept at reading tenor clef as well (and maybe even treble clef). High school players may not have learned other clefs yet, or may be slow at reading them, but a professional should have no problem.