There's always the option of renting too. If in doubt, rent a f/4 one weekend, and the 2.8 the next. Conditions will probably be different, but you'll get a feel for the difference in the lens anyway. Weight was never an issue for me being I went straight for the 2.8, and elected to have the IS just because of the amount of people upgrading from non IS and the possibility that I might want to do a portrait of my kids. I do portraits now, so the IS comes in handy. I'm not just a hobbyist any more, so my decisions are different for me now. But I was at one time, it just happened to develop into a business. But just so you know, my hobby started out taking shots of soccer games my wife coached and that our kids were in a long time ago. I'm still using the same lens I bought back then, the 2.8 IS, and no regrets what so ever. I had a budget back then too. But I'm a firm believer in buy once.
I started out sharing photos (giving away for free), then adding expense to these photos, and people were willing to buy. Now I charge for all my photos and have turned it into a business. It's a side business and I'm getting more and more gigs all the time. But it all started out with my 70-200 2.8 IS Mk I. So I'm sorry if I'm a little biased about this lens, but it's made me the photographer that I am today. It's my money maker. The OP is in a place I was once in, a hobbyist looking for good glass. And yeah, it's twice the price of a non IS 2.8, but like I said, try one out for size and you be the final judge. I did have the opportunity to use a 2.8 IS before I bought. As a matter of fact the photo that sold me on the 2.8 IS brought tears to my wifes eyes. She coached my son and he was a pill (along the same path as "don't teach your wife to golf", it never works out in the end), and after scoring his first goal, I caught this image:

IMAGE LINK: http://www.happyvalleyphotography.com …101336&k=kMSn5vx&lb=1&s=A
My wife was under the impression he was unhappy with soccer due to his antics during practice. He simply didn't want to listen to his mother. So when she saw how happy he actually was at this moment, she cried. That was the hook that got me.
Six years later, my equipment is payed for. I'm not saying that's where you'll be in six years, but I was where, you are now. It started as a hobby taking pictures of soccer. I opted for the 2.8 IS.
BTW, this particular photo was with a t/c on the 2.8 and this photo is 300mm 1/750 at f/6.7 on a 40D. It belonged to a friend that let me borrow his equipment. Good luck Silma in whatever you choose.