Right now I am thinking about to buy one or the other, so I am in a very similar situation with you.
I will explain the differences between each laptop and then comment on the pros and cons of each difference, if there is any.
1) First off, since you indicated in your post that you want to enjoy Blu-ray, you may wanna know that, in FW series you can add a mere Blu-ray reader drive for $150 OR, by paying an additional $150, a Blu-ray reader/writer (burner) drive. As for the Z series, you are not allowed to opt out the writer-- you can either add a Blu-ray writer and reader or will have to opt out for not any Blu-ray at all. Besides, I guess due to the fact that it will be harder to put a Blu-ray drive in such a small form-factor, the price tag is higher for the Z series. You have to pay $500 to add the afore-mentioned Blu-ray writer&reader drive, whereas such drive totals to $300 for FW series.
2) Second, the screen quality is different. FW accommodates either Xbrite-ECO or X-brite Hicolor, the latter of which is a better quality screen, whereas the former is said to be by some people crappy. You may wanna see the below link:
http://forum.notebookreview.com/showthread.php?t=18632
The thing is that, among the current pre-customized models that Sony offers, only the top-of-the-line model includes Hicolor, which retails, as of today, for $2150 and on sale on amazon.com for $2000. (Of course you can customize your own laptop if you don’t wanna buy this model.) The link is below:
http://www.amazon.com/Sony-VGN-FW198U-Processor-Blu-Ray-Ultimate/dp/B001A1QCGE/ref=sr_1_3?ie=UTF8&s=electronics&qid=1218508066&sr=8-3
I am assuming Sony will soon offer this Hicolor type screen in some new and not-yet-announced pre-configurations in near future. Also, there was a link that I cannot find now in which a notebook reseller mentions that he is corresponding with Sony and Sony will offer new configurations soon. In fact he was specifically referring to graphics card, but I guess Hicolor would be an option that Sony will also add to its future pre-configured FW laptops. BTW, you may wanna wait for that better graphics card, which is ATI HD 3650, if you like playing video games. The last thing that I will say about the screen quality is that one of the notable reviewers criticizes the quality of Hicolor. If I were you I would not care about what he says, because Sony is very famous with its vivid screens. I am not saying that he is telling a lie or something, but perhaps he is over-meticulous. Nonetheless, the link for his remarks, along with his review, is below:
http://www.notebookreview.com/default.asp?newsID=4524
As to the screen quality of Z series, they come either in so-called scratch-resistant Xbrite Duraview. No one knows whether it is better than Hicolor in terms of visual experience. But chances are Hicolor is vivider, and duraview is more durable, hence its -dura prefix I guess. Also, as far as I see from a video posted on the Internet and according to what the guy says in the video, the Z's screen glares. The link is below:
http://blog.laptopmag.com/hands-on-video-sony-vaio-z-series
3) Screen resolution:
Please read another answer I gave to someone's question:
https://answersrip.com/question/index?qid=20080810052444AA31QQ5&show=7#profile-info-BIXtbtasaa
If I were you I would not sacrifice from this 1900 x 1600 resolution. It makes FW series truly a movie-centric PC, and, for the first time, makes a 13 inch laptop's small screen usable (at least for me that is the case)
This resolution comes in all FW laptops. As for the Z series however, as of today among the 5 pre-configured models this resolution is available only for the top-of-the-line model, which retails for $4000! (Of course you can customize your own laptop if you don’t wanna buy this model)
4) You mention that you will attend college so you will intensively use the keyboard. You must consider that the keys of the keyboards of FW and Z series are quite different than most other laptops you will see in the market. Each key has a space between each other. Such space is even greater in Z series. Check the photos of two laptops from top angle;
http://www.notebookreview.com/default.asp?newsID=4524
http://www.sonystyle.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10551&storeId=10151&langId=-1&categoryId=8198552921644570897
You may either hate or love this style. So this is an important factor you need to consider.
5) Also you will need to take your laptop with you often to your classes. There is a considerable dimension difference as well as HUGE weight difference between FW and Z series laptops.
6) As to the graphics card, as I indicated above, FW will easily handle Blu-ray movies but will not be enough for gaming. It seems Z series can also handle gaming, unless you care about playing on a small screen. Also, Z series is using hybrid graphic cards—one for stamina and one for power. This is not a new development for Sony. The new thing is that, now you can switch between stamina and speed modes without booting the computer.
7) Huge price difference between FW and Z series. But I think Z series pays off its price tag b/c as I said, at last its high resolution makes that little screen usable. Besides, it is feather weight with plethora of features.
8) There is an optional, somewhat dual drive option of Z series, costing $1300, which I am sure of you would not need it.
MY OPINION: Now I will explain what I require in each computer, because I cannot speak on your behalf, and I have a PS3 and thus a Blu-ray already.
If I decide to buy FW series, then I want a) Blu-ray (reader only), despite I have a PS3, b) Hicolor screen, b/c FW series is created for movies. For me, if a FW does not include these features, then it will always be missing something.
If I decide to buy Z series, then, considering the high price tag as well as the absurdity that Blu-ray drive is only offered as a reader/writer combo and that I have a PS3, I can easily sacrifice Blu-ray drive. What I would not sacrifice on that computer is the high resolution.
The only thing that I can advise you is, I know the school is going to start soon, but wait for Z as well as new pre-configured models of FW. In the meantime, also the prices will also fall. Second advice: Do not buy any of them without going to a store and experiencing each form-factor. Remember what I said about the keyboards especially.
PS. If you wanna read the round up of the notable reviews of FW, check the below link:
http://laptopcom.blogspot.com/2008/08/sony-vaio-fw-reviews-round-up.html
Here are two good videos (not available on youtube) that you can see, respetively, FW and Z series:
http://blog.laptopmag.com/hands-on-video-sonys-vaio-fw-series
http://dfj-store.com/product_info.php?products_id=1226