I'll try and give you some numbers to work with, and I'll use read rates as the example to keep things simple. First off, most disks will have average reads of somewhere around 50MB/sec. USB 2.0 can handle 60MB/sec, FW400 49.152MB/sec, and FW800 can do 98.304MB/sec. eSATA can do 300MB/sec.
If you'd also like to access the drive on a windows machine, get one with USB 2.0. Most windows machines don't have Firewire 400, let alone Firewire 800 included.
You're going to see a 10MB/sec theoretical difference in transfer rates between the USB 2.0 and the FW400, which you'll probably never even notice. However, as others have said, the FW uses less CPU overhead, and isn't "bursty" like USB 2.0 can be.
eSATA isn't really that useful unless you need to get over FW800's 98.304MB/sec transfer rate hurdle (or you absolutely need S.M.A.R.T.), which puts you in the realm of editing high def 1080p 10bit uncompressed video (most likely less than 1% of the reading population on these forums).
One with both USB 2.0 and FW400 would probably suit your needs, now and in the future, and should be one of the cheaper options.
In summary:
-Get FW400 for video editing on a mac.
-Get FW400+USB 2.0 if you want to use the hd on a windows machine
-Get FW800 if you plan on doing hidef content, use high quality hard disks, or two-disk RAIDs. If you don't know if you need more than 50MB/sec transfer rates, then you probably DON'T.
-Get eSATA if you want to use four-disk RAIDs and need to edit the next documentary for IMAX
For the cost difference and what you're using it for, I wouldn't go with less than 200GB.
