Here are the differences as far as I can tell:
1. Graphics. T430 = NVS 5400M 1GB; X230 = Intel HD 4000. I hear the latter is quite capable, there are some benchmarks available on Notebookcheck. For your current usage the Intel would be fine, but it's not very future proof for newer games. Will you be upgrading again in 2 years?
2. Screen. Obviously you are losing the 1.5", but more importantly you are losing about 30% of desktop area with the lower resolution. To make this more difficult, the X230 is IPS. Not sure how picky you are with screen quality (to me it doesn't matter, a nice TN panel is fine) but the X230 will most likely be a lot better. I think the T430 will use T420 panels, so it would be a good idea to read some reviews there.
3. Ultrabay. Like mentioned above, unless you buy an ultrabay (~$200) or a USB drive you won't have the optical drive, a second HDD, or an ultrabay 3 cell battery. Your call..
4. Wifi. T430 = Intel 6300 = 3x3 antennas; X230 = Intel 6205 = 2x2 antennas. Theoretically with the right kind of 3x3 router you can achieve better speed and range, both are good cards however. If you get rid of the webcam you can get the 6300 with the X230.
5. Battery life. Not sure on this one, but I would think both machines (T430 w/ 9 cell; X230 w/ 6 cell) should achieve similar battery life. Once more reviews come out someone will surely test this.
6. Portability. X230 = more portable, slightly smaller power brick, system itself about 1lb less. I am perfectly fine carting around my slightly heavier T400 with the 90w adapter in a backpack, but if you have to carry tons of books etc. less is always better.
That's about it, hopefully that's helpful, although it probably just complicates matters.
I'm assuming these are school-issued laptops with no charge on your part? Or is the laptop yours to own? If you're buying, what are they quoting you for price?
nkPad T400: T9400, 8GB, LG WXGA+, Samsung 830 128GB + WD Scorpio Black 500GB, Intel 5300agn, Win7 Pro x64Others: IBM ThinkPad R40, Sony VAIO NR Series, HP TouchPad running CM9, Jailbroken iPod Touch 4G