Okay, I'm here to offer my opinion.
I am the proud owner of a 15" powerbook with combo drive with 512MB RAM. I've had it for about a month.
For those who mentioned above that are considering an iBook, my advice is that if you have the money and can afford it, get the 15" powerbook. IMO it is a better built unit than the iBook, and the general features are better all round - which I won't go into since you've all done your research. Even the hinge of the screen is built well - the joint spans nearly the length of the edge whereas in the iBook, the hinge is just 2 sections that are joined. I don't know if I'm clear in the explanation, but if you look at it you'll know what i mean.
Overall, I think the powerbook just looks so much better and is built with better materials. This keyboard is a joy to type on. However, if you're tight on money, the ibook is still a good buy. But like i said earlier, if you have the money and can afford it, the powerbook is a better choice and you will probably keep it longer because you won't find it slows down as quickly as the ibook when you're doing more tasks on it.
As to the 12" or the 15" - I would recommend the 15. I was considering the 12 before settling for the 15. My reason for that is because I think the 12 although very compact, screen real estate is lacking and I might regret it later. Weight difference is minimal. The 15 is thinner, and even though it's bigger, you won't really be carrying it around without a case so the extra size isn't going to make a huge difference when you're carrying a hand case. Weight would make a difference if you're lugging around something heavy but as I said weight difference is minimal. The LCD display on the 15 also looks crisper than the 12. it's more expensive, so again i say if you've got the money and can afford it, go for the 15.
As for the 2 standard options - I went for the 1.5Ghz and combo drive. I don't think the 0.17GHz extra speed is worth the extra money, and as for the superdrive, i won't be using the laptop for burning discs (in fact I am yet to burn a CD on here). I've still got my PC and it's so much cheaper to get a dvd burner drive for the PC. If I do for some reason want to author a dvd on the mac, I can always transfer it over to the PC for burning.
As for memory, I went with 512 because I don't do any extensive video or image editing so it's not going to require so much memory. 512 is running fine with no noticeable problems with what I do - internet, chatting, word, excel.
If later the need arises, can always add another 512 RAM. Proabably cheaper to get it from someone else too than direct from apple
