I read alot about the 2 lens, the reviews said that the 70-300 was a faster focusing lens. And boy does it ever. I don't have anything to compare it to, but I did shoot some pics last week. From my deck, I was able to pull the print off the stake that the funeral home gave us for the tree from Tammy's Dad a few years ago. That was at about 23-25 feet. The tag was about the size of a mailing label. I love the IS feature (Image Stabilizing). I got it on both lens. I love it but it's big.
I was a bit worried about the 70-300mm though. when I first started playing with it, it just would not auto focus. The motor would spin and spin, but the image would not clear. I had to actually sit down and look at it, turns out the AF was not switched on. I am worried though, I keep forgetting to lock the lens after use.
Working on finding a camera bag now though, I'm really fussy. I might take you up on that offer to try your 100mm though. I was looking at them 2 nights ago. It's tough to convince myself that now, right after that initial purchase, is a great time to run out and spend 400$ again.
Jason