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@Klackman, the main difference between the Osprey Ariel AG 55 and Osprey Ariel AG 65 will be the volume of gear each pack can carry, the 65 being able to carry a bit more. Typically the number in the name of a pack indicates the size of the pack's gear capacity. A great way to compare items side-by-side on our website is to select the 'compare' button under each item - you can see the specs of those 2 packs side-by-side here. With packs, when possible, we highly recommend getting fit to ensure the pack will rest correctly on your hips and shoulders - if you live near an REI, you can book a pack fit appointment here. Hope that helps!
For wilderness backpacking, generally a 55 L pack is meant for 1 to 3 day trips and a 65 L pack is meant for trips up to a week or so.
Generally you don't need to bring a lot more gear on a longer trip but you do need to bring more food and you may need to carry more water so this is only a broad guide. Whether or not you need the larger bag depends on how bulky your gear is. If you are transitioning from car camping and plan to use some of your existing gear you will probably find it won't fit in the smaller bag...why I returned a 55L for a 70L back in the day. Other reasons you may need a larger bag are
1. traveling in bear country where a bear can is required. It is generally more comfortable if the can fits inside the pack.
2. Winter camping which requires bulkier gear.
3. You have hobbies interests or particular needs that require gear that you want to take on particular trips so you want some flexibility.
4. You lead a group..eg kids, family or friends who don't have much gear...and you carry more of the shared gear.
The thing to check is that the larger pack can compress down sufficiently when you are carrying less gear so it still carries comfortably. How the compression straps work may vary with the capacity of the pack even though it has the same model name. The 55 and 65 ariels appear to work similarly but its worth checking.
One reason some people give for not getting a larger pack is that it tempts them to take stuff they don't need just because it fits and they just end up with a pack that is way to heavy. I find that a bit artificial and think it is better to have a larger pack but focus on base and total pack weight when pulling together your gear for a particular trip. Travel luggage scales are now quite cheap and are great for this.
If you are planning an extended thru hike different rules apply since you will be more concerned with efficiency. A smaller capacity lighter pack may be the better choice but that requires more thought.
Note that the capacity is different from the fit size (XS, S, M, L) which is the size of the harness for different size people determined by the distance between the belt and shoulder strap attachments. It is also different from the load range which is the amount of weight the backpack designed to comfortably carry. The 55 and 65 Ariels each come in XS S M which can be customized and it seems REI sells an Ariel 65 L but since they are of similar design they both have the same load range of 35-60# (according to the Osprey site)