External HDDs and upgrade path?

Firewire. It's still there.

And the connection also depends on what Mac you have.
There are several that could support eSATA.

 

external hdd's

Firewire is ok, I can't wait for USB3

 

If you look around I'd get one that has connections for both Firewire and USB2 on the back. Unless you get a portable 2.5in USB powered one a multi connection in the back is not uncommon. My WestDigital had Firewire, USB2, eSata, and UltraWide SCSI(Just kidding). No really, big drives should have more then one connection.

 

If I were choosing a new external, and eSATA wasn't an option on my particular mac machine, I would go for firewire 800. It is almost as fast as eSATA.

In a year or two either USB 3.0 or Lightpeak is going to start becoming more widely adopted by industry (to me, Lightpeak seems like the more promising technology).

 

If it is a single drive, I would get FireWire 800 over anything else.
For more than one drive, I suggest using eSATA.
If you really need fast, portable, and not to worry about upgrade it, try this
http://www.timelinedigitalinc.com/iostudio-pcie-expansion-pcie-card-standard-length-p-848.html
Fully compatible with Fusion I/O and the speed is over 1GB/s