I'm an Infuse 4G owner and I would say if you can handle the extra $100, just go with the Galaxy SII, it's totally worth it. The Infuse was released at a very odd time when the Galaxy SII was already being flaunted as Samsung's flagship phone in Korea/UK. It lacks many of the internals that the Galaxy SII has (Infuse only has single core, less ram, slower processor, etc. all of which honestly, you may or may not notice on your daily use depending on what you do).
Most importantly, the Galaxy SII is Samsung's new flagship phone. That means it has the greatest user base of all Samsung phones that have been released recently (maybe second to the Nexus), so you know that the SII will be given the most attention as far as updates/bug-fixes, etc.
The Infuse has a promised Gingerbread update and it is indeed coming very soon (within 2 weeks or so). However, it most likely will not get Ice Cream Sandwich, which will be coming on the SII.
As far as SII vs. Skyrocket, I personally prefer the SII. The Skyrocket once again is slightly a niche phone and will most likely not receive as much attention from Samsung as the SII. Skyrocket/Infuse both have 4.5" screens and while it's nice and big, it has the same pixel density as the SII, so you'll have a bit more noticeable pixels when viewing. But I've played around with both an SII and an Infuse and a Skyrocket, and honestly, the SII's 4.3" screen seems to be the sweet spot for me as far as being both large enough and still being ergonomic in one hand.
I have pretty big hands and when I use my Infuse, I still have to stretch a tiny bit when using with one hand. This didn't happen when I was playing with the SII.
Battery life is ok. I charge every night and my battery lasts me usually until from 8 AM to 8 pm ish. That's when the battery starts to go yellow, and red around 10 pm.
The Super AMOLED + screen is much better than the Captivate's Super AMOLED screen though. You really just have to see them side by side to realize how much better it is.
With all this into account, I think the most important question is what you do with your phone. If you are a basic user (i.e. email, texting, calling, browsing, etc.), I'd just stick with the Captivate if you're worried about the money. However, if you watch videos on your phone, play games, and also use it for taking a lot of pictures, I think it's worth it to pay the extra $100 or so and go for the Galaxy SII (or the Infuse, if you want to settle on that instead). You are going to be using that phone for 2 years from now on - in that sense, especially if you think about just how much your monthly data fees surmount to, it's just worth paying that extra bit for a better 2 years of use.