|That's kind of a hard one. My first son was a super-chunk and outgrew the S Sandys around 18/19 lbs, around 4.5 months. My second son was small and skinny, and was able to wear them over 20 lbs, which for him was over a year. Chances are that you might be able to make the L work, but they would surely look very bulky. I second pichou's suggestion, since at this point, the only way to know for sure is to try. ME has a great intro package, and they'd do it with Sandys if you call them. I'm not sure if they'd do it twice (i.e. one for a S and one for a L Sandys, though you could ask), but if you used the intro pack to get one with a cover and then just bought the other, you'd still do okay.
If the S fits best, some thoughts: If you think you'll be having more children to wear the S Sandys, then they might be worth the investment, even if they aren't worn for that long. Or consider looking on the buy/sell forum to see if anyone is selling some S that you can use for a bit and then sell on.
Another thought: the OneSize is a different style of diaper, but one of its benefits is that if you bought some now you wouldn't have to buy another size later on. You might really want the Sandys, and they are a great diaper, but if you're not completely fixed on the Sandys, it might be worth giving the OS a look.