This is a "belly button" issue. Everyone has one, as they will have an opinion. In the end, the decision is a personal one, for you to make. Many folks have stated worthwhile positions above. All are valid. There is no "correct" answer.
My "two cents worth" is as follows:
In 2013, I started out using a 2-liter Osprey brand water bladder system designed to work with my Osprey Kestrel 46-liter rucksack. At the time, it seemed such a logical solution. However, I learned from experience that I am simply not going to invest the daily maintenance effort required to keep the bladder clean, maintained, and full of potable water.
PLUS, even when the system was completely empty of water, it still weighed some 11 ounces or 312 grams. By comparison, each disposable, recyclable half-liter bottles weighed about 1 ounce. Every ounce and gram counts. They do add up!
By the time I walked from St. Jean Pied de Port to Pamplona, the bladder went in the mail to Ivar. I later sold it.
For the remainder of that Camino Frances
, the next year (same route), my 2015 Camino from Porto, and this year's planned stroll from Madrid, I used and will plan to use 4 - .5 liter water bottles that I clip to the FRONT of my rucksack harness. In 2013, I went from the bladder system to bottles in rucksack side pockets. THAT, as others have correctly pointed out, is a huge logistical hassle.
Moreover, in 2013, I observed pilgrims from New Zealand with Aarn rucksacks, having two large, soft, "Pamela's" attached to the front of their unique harnesses. The pockets are actually called "balance bags" and only mount to Aarn rucksack harnesses. The nickname is in honor of a well-endowed actress in the US "Baywatch" TV series from the 1990s.
What these folks discovered in their Aarn rucksacks with the front balance pockets, was the benefit of shifting weight from the rear to the front of the rucksack harness. Shifting even one kilo from rear to front affected the balance and perceived weight of everything you were carrying. I note in passing that one-liter of water weighs one-kilo. Thus four half-liter bottles would weight two-kilos. Thus, I argue that moving this much weight from the rear or side pockets to the front of your harness is a good thing.
So, starting in 2014, I began to use stainless (inox) clips that grip the bottle neck and provide a clip for hanging on the sternum strap or other hanging points afforded on the front harness, or on my waist belt. My clips come from Nite-Ize, but I have seen others on Amazon.com. To secure them to the bottles better, I use #18 plumbing silicone "O"rings from my local DIY store.
Also, another pilgrim, last year, posted a brilliant method for making hanging mounts from very thin nylon cording. He tied the cord around the plastic bottle neck rim, and fabricated a method for connecting the homemade rig to the front of his rucksack harness. I recall that, at the time, I thought the idea was brilliant. However, I cannot find the posting. If anyone does know of it, please copy the link here.
The advantage of my clips is that one can get a drink using one hand. I do not know of any other method that is as convenient. The only down-side I found is that sometimes, if you bend over, the hanging bottles can slide free, especially if they are heavy with water. I have since started using rubber bands to create a "link" to hold the bottles close-in to prevent this from happening...it does work.
My bottles are readily available, they come FREE with spring or mineral water in almost any tienda or cafe. When the bottles get grungy, discard them, recycling is best, and replace accordingly. Four Vittel brand water bottle usually last me all the way to Santiago. I prefer Vittel bottles, so far, as they are stiff, well balanced, and have a good grip. They are good for a month with daily rinsing under a potable tap.
The single BEST reason why I submit that several small water bottles are superior to one large bladder, IMHO is that you can carry different "mixes" in different bottles at the same time. For example, I must use powdered protein as I have a bariatric lap band on my stomach. So, I start my day with two bottles containing protein powder in water, one with powdered coffee added for "punch." One bottle has an electrolyte tablet dissolved in the water to make an energy drink, and the fourth bottle contains plain water. As the bottles are emptied, they are rinsed and refilled with plain water from a potable source. I will buy a liter or larger bottle of water on occasion to refill my bottles and share the excess with others. As time permits, I can add electrolyte tablets and / or protein powder as needed to replenish those needs during the day.
Trying to do this with a large water bladder would be inconvenient. It would have to be cleaned every time, every day. Too much work for me...
I hope this helps.