About to transfer 420GB from one USB HD to another...

As the speed limit will be the USB speed and the NTFS format, you can safely use USB as interface. I haven't copied that much data from an NTFS volume yet, but maybe just do it in chunks, like if you have five folders on the root level of that NTFS HDD, do it one folder at a time, though I often copy hundreds of GBs from HFS to HFS volumes and don't really experience any problems with that.

Use the firewire port on the one, having the two drives using different buses should speed it up a little.

What I would do is start the transfer and leave it on overnight. No real reason to break up the transfer in chunks. It is not as if the files on the original drive is being modified if something is to go wrong.

I second the "start it and leave it overnight" suggestion--I just moved about 1TB from my old server drive to a new replacement, which took about 7 hours, but I just got it started and let it run while I was at work for the day. Unless something goes wrong with the process and it hangs, it'll be done when you wake up/get home. And even if it does hang, it won't slow it down any overall, and it's not like it'll break anything. It'd be particularly useful to set and forget because in my experience the NTFS driver is pretty slow, so it may take quite a while.

I suppose if you were getting the occasional problematic file that refuses to copy due to name weirdness or something, doing it in chunks might help you narrow down where it is (if it's invisible), but going from NTFS to HFS probably won't have any issue.

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Remember to "copy" and not "move", also!