I consider myself reasonably well travelled and have done a big circle around the world without having fallen off the edge. I normally travel with an 80 litre backpack and carry a small daypack which holds tickets, passport, coat, brolly, some water, and my netbook entertainment system.
I rarely travel with a case. A case is good when you are carrying lots of clothing, and in particular when you are carrying suits or other clothing which should not be crumpled. A case will carry perhaps twice that of a backpack, but I find them to be too cumbersome. There is a move to small dragable cases which can be taken on board a flight, but I find these slightly too small. One day I will fall over one of those.
As examples of the advantage of a backpack over a case consider these.
If I am going through a busy airport (Heathrow) I can walk at a fast pace through crowds and up escalators, jump on and off trains, and will not obstruct people on the way. With a case I find this opposite. On the tube and on buses a case always get in the way.
I clearly remember having a case ripped out of my hand by an over enthusiastic “porter” at a foreign airport. I thought he was going to steal it, but he took it a few hundred yards to a waiting transfer bus and stuck out his hand. He could not do that with a backpack.
On several occasions I have had to walk several miles to get a connection, as either trains were not running or it was easiest to do so. With a backpack you are away and do not have the issue of an aching arm dragging a heavy case.
I find a backpack easier to stuff. On the outward journey I usually have ample space, on the return the backpack it loaded to the gunnels with all kinds of things.
One minor disadvantage of a backpack is at airport check ins. Most insist non standard luggage goes to a security check in rather than down the chute in the normal way.
So for me a backpack wins hands down over a case for a long stay holiday.
Ensure your clothing is packed in a plastic liner. When it rains clothing stays dry.
I carry a simple bike lock so that if your pack is out of sight you can secure it slightly better.
Carry some bin liners for packing items on the return journey.
Anything electrical should have its own separate plastic bag.
Make sure heavy stuff is at the bottom, easier to carry.
Make sure the carrying straps are correctly adjusted, otherwise your shoulders may hurt or the pack will be wonky.
Tidy away any loose straps to avoid entanglement in machinery.
Make your pack distinctive so it is easily identified.
Have address labels tucked out of sight. If it is lost they will do their job, and be away from prying eyes.