The 8100 with 15" is huge - the anti-laptop.
<< Additionally, the 8100 has a firewire port which may be useless to many, but if you're doing video editing, it can be quite a nice feature. >>
The firewire port may be more useless than initially one may think. It's a non-powered port, which means it won't support any peripherals that require power, like a firewire drive or media reader, etc, unless you buy an external powered firewire hub (which kinda defeats the point of portable firewire peripherals). (It will work with a firewire camcorder though since the camcorder will have its own power.)
By the way, so far I have not been impressed by laptop nVidia drivers when using the VGA-out port. Not very user-friendly and not reliable for autodetection and configuration for external displays. (This is specifically with the 8100 - several of my colleagues have them. I dunno about the 4100 with nVidia though.)
Whatever you do, get it with a built-in modem port and built-in Ethernet port. PCMCIA Ethernet is suck.
Neither is my first choice (see my rigs), but out of those two I'd get:
Inspiron 4100 with 1 GHz PIII.
256 RAM, preferably on one SODIMM.
20+ GB drive
Radeon
14" screen at 1400x1050. (1600x1200 not advised)
Integrated Ethernet and modem
CDRW or else CDRW/DVD
Note it doesn't have Firewire, but like I said, the 8100 only has half-@ssed firewire anyway.