I'm in a similar situation like you, the only difference being that I don't have any backache.... All my bags seem to be big or heavy, which is why I went in to LV last week looking for something specifically light! I have an almost 3 year-old constantly by my side, and carrying him plus grocery shopping etc. has made it impossible for me to handle any of my bags, really... So I bought the mono Neverfull MM and azur speedy 30 and couldn't believe how LIGHT they are!
I got to try on the Bellevue PM in bleu nuit as well, and I must say that it's actually a wee bit on the heavy side when empty (let alone with all my stuff in!). Nevertheless, I might still buy it towards the end of the year because it's sooooo pretty and chic!
Anyway, I would highly suggest the Neverfull as it's quite light but very roomy, and provides a great alternative to your speedy! Good luck deciding..