WELCOME TO THE BOARDS BOSHWOK!!!!
Okay, well those packs I'm sure will be fine, but here are some suggestions. I have been on the backpack hunt myself for the last couple of months, and I finally bought mine. Its from MEC (I live in Canada) and its called the Walkabout
This has all the features that I could want, it really depends on what YOU are looking for. This pack unzips like a suitcase. Those packs that you linked to look like the top loading kind, which is fine, but can be a really pain if you have to be digging around to find something. The packs that you can zip open the whole front really eases things, and you can get at everything you need easily.
The zip-off daypack is a feature that you have to decide on for yourself. I opted for it, it is a really useful feature I'm sure. It really depends on how big your pack is to start. If you are only going for three weeks, then you could probably get away with having a smaller pack that you just carry everything around in it with you.
Another interesting and useful feature is a cover to zip over all the straps and what not. That MEC pack I bought is pretty neat because this whole flap zips over the entire back to hide all the shoulder and waist straps so they don't get caught and wrecked when being loaded onto the airplane etc.
Okay, well there are a few suggestions....it is definitely worth shopping around and TRYING THEM ON. You don't want to buy a pack over the internet only to realize that it is uncomfortable...that could make for a long trip!!!