If you will potentially need that extra capacity on a future trip, then definitely get the 75L pack. If you're doing it just because it doesn't seem like much extra weight, even if you don't expect to ever need it, I would go for the 65L pack. That would also curb the temptation to overpack.
Trips of 5 days or more usually call for packs of 70 liters or more. These are also usually the preferred choice for winter treks lasting more than 1 night. (Larger packs can more comfortably accommodate extra clothing, a warmer sleeping bag and a 4-season tent, which typically includes extra poles.)
The 40L will max out carry on dimensions on most airlines so you never have to check a bag. For stricter airlines, like RyanAir, 30L will be a better choice.
Regular backpackers have packs that weigh about 30 lbs. They're generally newcomers to the hiking hobby, and they have a tendency to overpack. If you're a newcomer going backpacking for the first time, it's a good idea to never exceed 40 lbs and go with an experienced friend or a group of hikers.
Their capacity lets you carry an extra layer, food and gear for day trips. 21–35 liters: This is the sweet spot for most hiking and travel daypacks. There's enough capacity to hold food, clothing and some extras, like a camera and a book.
Lightweight backpack for outdoor, Ultralight, only 2.1 pounds for 60L large capacity,reducing the backpack loading weight so that you can carry more other items. The maximum loading bearing is 33lb.
A 50L+ backpack is a great friend to those travelling for a few weeks. These bags are spacious and can accommodate a number of outfit changes before you ever have to visit a laundromat. But don't be tempted to fill your pack to its capacity before you've even left - remember to save space for souvenirs!
A loaded backpacking pack should not weigh more than about 20 percent of your body weight. (If you weigh 150 pounds, your pack should not exceed 30 pounds for backpacking.) A loaded day hiking pack should not weigh more than about 10 percent of your body weight.
The design of the 70L is perfectly suited for airplane carry-on or check in, and the features of the 70L are perfect for any adventure once you arrive at your destination. Being able to compress the 70L allows you to use it as carry-on for all major airlines in the US and Canada.