Professors vs. Grad Students: I took both my FWSs with professors instead of graduate students and based on vicarious comparisons, I think that was a sound choice. Some graduate students are promising as potential professors, but most of them will be stepping out of their academic comfort zones for the first time in terms of the amount, difficulty, and coverage of the material assigned. Many of them will possess the knowledge required to lead the seminar, but not the pedagogical experience to tailor it to the average freshman. With professors, their critical eye when it comes to grading papers or their sensitivity to your comments - frankly, a group of freshmen sound nothing like a symposium of specialists - and their ability to moderate discussions is often better honed. Of course, some professors bring their own brand of whimsy to the classroom, but professors who teach FWSs are not known for being exceptionally tough graders, ineffective teachers or disagreeable people (with few exceptions). Also, If you have the chance to take a seminar with one of Cornell's 'elite' professors, grab it, particularly if the seminar description looks interesting. I took my first FWS with a notable professor who's published seminal papers in her field, and I'm sure it'll be one of my most memorable academic experiences at Cornell. Such professors have probably experienced the whole gamut of teaching environments, and they're likely to thrive in the seminar room - their main pupils (graduate students) tend to be taught exclusively in such settings.
Timing: It's self-evident that you should pick classes that fit into your schedule and which are least disruptive to the sleep cycle you'll fall into when you get here, but especially if you're a morning person, I'd encourage you to sign up for a seminar early in the day. I chose 8.40 seminars both semesters, and found that most of the 16-17 other people I was sharing the classroom with were more enthusiastic, more articulate and somewhat sharper than what I've come to see as the 'average' student at Cornell. I don't think this was a coincidence.
Content: FWSs tend to come in two 'flavors'. Instructors who subscribe to the Knight Institute's way of doing things will place a heavier focus on the technical and procedural aspects of writing and devote more time (and points) to correcting your writing style, structure, grammar, outlining, and so on. The topic of the seminar in such FWSs tends to be a 'vehicle' for teaching the more important 'art of writing'. Personally, I don't believe writing can be taught in a classroom (at any rate, it can't be taught in four months) and I'd avoid this sort of class like the plague - even if it means trying to add/drop your way out. The other kind of FWS tends to be content-driven; some of these are influenced by the academic traditions of a certain discipline (e.g. English, History), while others are designed with the awareness that most students aren't seeking to specialize in a particular area anytime in the near future - in any case, both are valuable ways to gain an appreciation of a fresh body of knowledge.
I'd use these three broad considerations in picking FWSs but others things you might want to consider are: writing to your instructor for a rough course outline/syllabus (reduces the rude shock of finding that course descriptions aren't always very faithful to what you'll actually encounter), asking about how much work the instructor projects, finding out if that seminar has been taught before (and by the same person), etc. Use the same caution in picking all of your five choices; I've known far too many people who've ended up in their 4th/5th option only to hate themselves for the rest of the semester.
Edit: I'd avoid many of the English FWSs for the above reasons - many are taught by graduate students, in a technique-focused manner.