saamI think worth upgrading. Rockets do 90 damage to sentries, level 2 sentries have 180 HP and die to two rockets, level 3s have 216 and thus take another rocket.
it just depends on what you're using them for
level 3s are better at soaking ubers.
level 2s are better at catching backcaps/flanks, while also being good at soaking ubers.