True, i noticed that most people went assassin after reaching 31, although i saw plenty of rangers.
i reached lvl 37 ranger myself and can say that up to that point the only difference between assassin and ranger is the skills(spells) u get, u dot get any direct +% to range attack, most of the ranger skills are ranged, though you get 2 melee attacks which make plenty of damage plus one stuns, so u have to keep you distance from enemies more than assassin...
I also noticed that after lvl 37 there was change in gear, up to that point my sprite made ranger/assassin gear but after that the sets were only for assassin or ranger, i maybe wrong here now, but imo the ranger gear was more +agi and assassins +str... and this game is gear oriented, so im guessing, but i think that's were the biggest difference comes in
but there are probably people who know that better x)
im thinking the most noticeable change comes after 40+