Personally, I didn't know it was possible to boot from a USB device, at least not on all laptops. I would check that out first.
If you can, you'd have to base your decision on how fast you can get a PCMCIA card. USB 2.0 runs at 480mb/s. The more money you pump into a PCMCIA card (assuming you have a slot that can handle the same speeds) you can get a very fast PCMCIA card, which can potentially be faster than the USB connection.
If you have one of those external laptop CD-ROM drives, just use that.
I hope you don't have a lot of these laptops...or yer gonna be hurt'n. If that's the case, I'd be more inclined to buy a laptop HDD imager than fool around individually.