The lower the reed number, the less stiff the reed, and everything else being equal, the easier to get a tone from it, the easier to control it, and the longer you can play before your lip gives out. However, the stiffer the reed, the more projection, and if you're up to it, the sharper your control. The thing is, everything is never equal--factors such as how developed the player's emboucher is, the facing and tip size of the mpc, even the material of the mpc can all change the formula. For example, I have a pretty hard time controlling my hard rubber Berg Larson 105/2 with a 2.5 Vandoren ZZ on my tenor, but an easier time controlling my metal Otto Link STM 7 with same reed, even though both pieces are comparable in tip opening. Consistent with that, the Berg is a lot brighter, and on the 10M has killer projection. The Link is darker, and contrary to expectation, does not scream like the Berg. But I play the Link more often, because given my emboucher, with the same reed it's easier. My understanding is that in general, the strength and quality of your emboucher is the critical factor---the stronger and more develped, the harder the reed you can successfully use, and the wider the tip opening you can be comfortable with on your mpc. The individual player's optimum ratio of tip size to reed strength can be adjusted: smaller tip size, harder reed (larger number), bigger tip size, softer reed (lower number). More experienced players may have better and more accurate info on this, but that's my take on it. Hope it's helpful.