I know Clay is partial to the 49t and he is a much faster rider than myself, this is just my $.02
I am not fast by any means, I ride B hare scrambles, but I have ridden quite a few GasGas setups whether it is suspension or gearing or exhausts or bars, due to working with Mark @ GoFasters for a year and a half. I personally believe there is not a perfect sprocket setup for a course, I think a lot of the gearing choices should be made depending on how fast one rides, and it may even go one further and say how fast one rides certain types of terrain, or even the mindset of how one rides. For only $65 for a Renthal sprocket it may be worth owning both a 48t and a 49t.
I personally run a 49t but I am going back to a 48t. Main reason that I went to the 49t I was trying to get better power out of the corners and make it easier on myself in the tight stuff.
I found that sometimes in the tight stuff I really didn't need snappier power which i got with the 49, almost seemed like I got in more trouble with it and I am a 240lb guy without gear and I ride a 2011 300.
An example would be on a course with many sandy corners and all the straights are short, it would seem that bumping up the rear sprocket to combat the loss of power and since you wouldn't need the top speed in that situation.
I find exactly the opposite in that situation, with a sandy corner I would rather hook up and lug my 300 without my arms being jerked coming out of a corner. Now if I toned it down, maybe that 49t would be better.
With the tight stuff, I actually prefer the 48t. The main reason for that is that I hate shifting in the tight stuff making either 1st gear taller or 2nd gear wider, again I would rather lug the 300 and maintain traction than to have my arms jerked in the tight trees, especially where I like to maintain stamina to keep a good speed.
One important thing to consider, a 49t will make your gears narrower and on a 300 that is not really what you should be shooting for. With a 250 maybe. I don't think you can go wrong with a 48t or the 49t if most of your riding is done at low rpms the 49t wouldn't be a bad way to go but if you are pushing the bike into the mid range rpms the 48 is very tough to beat in nearly any situation. I also feel like the bike is tougher to bump start with a 49 on as well.