Ruck Sack

So I thought i would start searching for a day sack for the trek, as I am supposed to be wearing it when training. Wow what choice, and where to start. Whilst the necessity is for carrying stuff for the day, including some additional clothing in case it gets colder, or I have to remove a couple of layers, then where to start. 30 litre is a recommended size, but it appears there are numerous models. Now being new to this hiking, I don't want to turn up with something inappropriate, the one pictured above good stand out a little.

It is amazing how much different and quite often conflicting advice there is on the subject. If you have any advice please post a comment, as right now I am still browsing.