Expandable slot vs. Firewire - need an explanation

I'm having trouble deciding between the Macbook Pro 15" and 17" for video editing and a mobile student computer. People in favor of the 17" argue that the higher resolution, bigger screen, and expandable slot make the increased pricetag and size worth it, whereas others argue for the 15" reasoning that it's cheaper, more convenient, and I can always plug it into an external monitor anyway.

For me, the main issue seems to be the expandable slot. I've been told that it's a necessity for running an external HDD effectively (until Thunderbolt opens some more options). Yet, I was recently told that I could simply connect an external HDD to the 15" through Firewire - because the 15" has a Firewire port.

Could someone explain the difference to me between connecting via Firewire vs. the expandable slot on the 17"? Why is it argued that the expandable slot on the 17" is so important for video editing?

Thanks.

It would be built in to the computer. It would be more convenient, and it wouldn't a data stress on the cable.

I'm aiming for Mac because (a) NYU Tisch supports them and (b) it's required for FCP.

Sort of. To clarify, I was referring to the ExpressCard/34 slot. I was wondering what the advantages to this slot were to justify the purchase of the 17" - particularly in reference to external HDD use - and whether Firewire could get me similar results.

I see no point. As time passes, you will start to see adapters for thunderbolt that will allow you to connect external devices like USB3 or eSata. Ergo I see no good reason to go with the 17" unless you really want the bigger screen.

And no dalauder, Mac doesn't have USB3. They want to sell their thunderpolt aka. lightpeak. Yes it sucks but oh well. At least in test thunderbolt was substantially faster than eSata. Now thunderbolt just needs some devices and/or adapters to actually be useful.

Don't get me wrong it's fast alright, just missing some hardware. Time will change that though.

In terms of speed, you won't go faster working with an external hard drive (specific exceptions not worth discussing exist). And since you'll only be using it for backup, the USB 2.0 or Firewire external HDDs will work fine for that. So go with the 15.6" laptop--especially to save $500.

$543 buys a new light gaming laptop (i5-2410M w/ Radeon 6630M). $500 buys a new 50" TV on Black Friday or other great sales. It's worth a lot to save that kind of money.