Yes there are several TabletPC Forums better equipped to answer this question, but you can't just compare NITs. There are also several technologies in use and being introduced this year that make those kinds of comparisons obsolete.
Your best bet is to demo several Tablets under the conditions you anticipate using it. And decide for yourself what trade-offs your willing to live with, i.e., price/screen size/prosessor/screen resolution/etc. You may find that the NITs is less important than the glareshield or polarization or some other factor not considered before demoing a few.
Personnally, I'm extremely pleased with my front-lit transflexive Fujitsu. And considering that I knew I would need 2 (one in-house for development and one in-car for use), I knew price was going to be an important factor in my choice. My tablet is my carputer for three different cars and I run as many as 8 USB devices so STANDBY and HIBERNATE friendly USB 2.0 controller was also a prime concern.
Screen readability in direct sunlight with the Fujitsu is less a problem than screen dimming for night use. Fujitsu, in their ultimate wisdom, didn't provide a command line nor direct access to their dimming utility, so I had to create my own. 650NITs would make your whole car glow at night!