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It is mostly preference but you might prefer a pure top loading pack in wet conditions and for transporting specialized bulkier gear for activities like climbing where opening side panels might cause a disastrous gear loss.
Front loading packs are generally top loading packs with additional opening panels. They are more convenient to organize stuff and find things. But...they have more zippers and dodads that can leak and fail. They have more stitch lines that can leak unless sealed. Zippers, stitch lines, sealant and pockets add weight so they tend to be heavier for the same capacity.
A pure top loading pack can be made lighter, be more reliable and less leaky. You just have to unload everything to find what you need.
Many people like multi-pocket front opening packs for the organization they provide. Other, generally more experienced people might prefer a pure top loading pack because they have their gear dialed to be efficient, always organize it in the same way and like the simplicity, the robustness and the weight savings.