As you have probably figured out by now, and based on the Original Post, PALS/MOLLE webbing adds configurability while exacting a small price in the weight of the PALS/MOLLE webbing alone.

I would say that whatever bladder carrier/light pack you choose must include both a sternum strap and a belly band.  Virtually every Camelbak Milsurp bladder carrier I've seen has a way to attach a belly band, which stabilizes the lower portion of the bladder carrier.  Mostly it's a channel through the fabric on the ventral side of the carrier.  BTW, most milsurp bladder carriers have an extra layer of fabric on the ventral side of the bladder carrier.  If you choose, this is an excellent place to insert some 1/4" tick closed cell foam (like from an old GI sleeping pad) to insulate the carrier from your body.  If you're running, a belly band is essential, lest the bladder carrier wear a hole in your back because it's moving side-to-side every pace.  Basically a 1" nylon strap and a QD buckle with strap adjustment on both sides of the buckle.  The sternum strap is essential to keep the shoulder straps in the right place on your body, and so avoid fussing with them every few feet of travel. 

While I don't suggest it, on account of price (the quality is GTG), the Source Commander pictured above shows that it has both a belly band and sternum strap.

 

That certainly might be a body issue, or simply a matter of personal preference.