Of course it's about personal comfort
What's also important is how secure your kit is to the ruck.
I remember in basic that damn dangling valise bag, exacerbating the weight of the ruck every time it bounced off of my butt.
The way I see it, you got two options:
1. Get some of those cargo straps and make that valise, to quote a Sgt, "one with the ruck." (i.e. sinch it in tight)
2. Move the valise to the top of the ruck so that it is above your center of gravity so it doesn't feel like it's pulling backwards
If you go with #2, make sure your valise does not hamper the movement of your head. You're already a hunch back with the ruck, no need to add injury to insult by forcing your neck forward in agany for +2 hours. (P.S. I appologize if I offended any hunchbacks.)
I personally have two valises; the issued one and a wheeler's valise. The wheeler holds tons more but is substantially wider and not ideal for ruck marches or working in the woods. But since my role in the field is usually stationary, it comes in handy for stuffing my second pair of boots, cleaning kit, washbasin, pillow, full sleeping bag (liner, inner, outer + bivy bag), ranger blanket, ground sheet and a dozen clowns. I still use the issued bag to stuff in 'just the essentials' for ruck marches.