what size did you get and why? i'm looking at possibly saving for new luggage for my birthday. i take mostly 1-3 day trips but do have a week long vacation every year or so. i can't decide between the 45, 50, and 55. no 60 because i would absolutely NEVER check LV luggage. leaning towards the 50, but sometimes i think it's too small, other times i think it's too big. also, do you use your shoulder strap? thanks!