Unless you have a specific sound concept in mind that you can achieve easier with a harder reed, what's the point of moving up? As others have said already. When you are a beginner just stick the that medium size reed. When you get a clear picture of the sound you want you can either move up, stay there or go softer. Yes softer doesn't mean easier. You are not lifting weights with your lips, you are developing a steady airstream and a " personal voicing" on a every register ( even individual notes) of the horn. Getting a great sound is hard enough without adding extra things that might just get in your way. I ( like many of us) can play perfectly on a 4 on my set up, but you know what? I hate the extra dry- asthmatic sound I have to fight specially on that middle D and in the low register when I use hard reeds. Some people might love it, but I truly hate it. So it's not a GOAL to play this or that reed size. It's just playing THE ONE that doesn't get in YOUR way, so you don't even have to think about the reed. There are so many variables that influence your choice of reed, without including the obvious set up. For an instance if you are a naturally bright player and you are aiming for a darker sound perhaps a slightly harder reed might make it easier and it works the other way round. On the other hand if you are a darker player you don't need to change anything. Please keep in mind that you have to darken/brighten your tone on your own. The reed helps but it doesn't do it for you.