If you are going to use the EHD as a storage drive for files or backups then Thunderbolt will provide the fastest performance possible. However you need a mid 2011 iMac to use it as these are the first iMac's to have Thunderbolt! I don't believe you have that new an iMac! The EHD's I normally suggest are:
G-Tech G-DRIVE series
LaCie d2 Quadra series
OWC Mercury Elite Pro series
All are similar in features and build quality. They all have Firewire & Firewire 800 connections which for pre-2011 iMac's is the quickest bus there is for EHD's. You need to check the specs on your iMac to see what connections it has. If you don't even have FW800 then while these EHD's would work fine they may be slight overkill however they will prepare you when you upgrade to a newer Mac.
Personally I own 4 of the LaCie's however any would be welcome on my desk. I have 3 daisy chained together via FW800 on one machine and the 4th is a TM backup for my MBA.