I have had both sizes...the petit is too large for an everyday bag, (at least it was for me) but was great for a day out with the kids, anytime I needed to pack extra clothes/bottles for them etc...
The large size is HUGE. I bought the large first, thinking that it could be used for the above, but it was WAY too big. It is definitely a luggage piece. I think it would make a perfect carryon, but you wouldn't get much use out of it if you're not travelling.
I LOVE the randonnee...I think the petit is my alltime favorite LV bag, it just wasn't functional for me. (The farthest I travel is to the grocery store...lol)