1, 50 liters or less You want a versatile bag that can be used as hand luggage
.
2, Top loader or a suitcase style zipper style suitcase I prefer the quick access to my stuff with the side zipper style.
3, Internal frame or not to internal frame all depends on you and weather you will be going long distances on foot with your backpack.
4, Suitcase or backpack? I say backpack.
5, multiple compartments will help you keep organised.
6, Rain cover or not to rain cover I hardly ever use mine but when I need it it's very handy.
7, pad lockable zips won't to thieves but its a good to have them to stop quick fingers.
8, Pick a good colour because your bag will get dirty Alys old red backpack looked like it had been ran over by a lorry.
9, , Don't pick a bag to look cool pick a bag for its features.
10, When buying a backpack have it fitted and filled with weight to making sure it sits in the right position on your back.