It depends, because if your drive is a 5400rpm (like the one I tested) there will probably be no difference between eSATA and FireWire (the drive I tested tops at 40MB/s which is 40% of FW's theoretical speed (I guess that at least 40% of its speed is achievable) however if the drive is 7200rpm it will probably be faster (again depending on the particular model, how much cache the drive has, and how big it is) in eSATA (however there should not be a big difference unless the drive is really fast (e.g. WD raptor or 1.5 seagate or any kind of RAID 0 config, or well offcourse SSD drives.)
But all that speed thing is pointless, what really matter is how the comfort of using either interface. Let me explain... USB for 2.5" drives will power the drive, which means 1 cable thats all, FW should also do the same (it is supposed to provide 45W of power and up to 30V so in theory it should also power 3.5" drives without external adaptor, though I have never owned a FW drive so I cannot say) eSATA in the other side it is much faster indeed than USB and for most HDD is slightly faster or faster than FW however eSATA does need external power adaptor regardless of the HDD (both 2.5 and 3.5 needs power adaptor) also the eSATA cable is not very flexible and so far the shorter I have seen is 3FT which means it will put considerable ammount of stress on your laptop and drive (enough to move lightweight enclosures) furthermore while eSATA and HDDs are cheaper than FW's you have to take into account you will need also the adapter (expresscard) which can be $15+ (sometimes even in the $20s and $30s depending on model)
So its all up to your needs, if your HDD is 5400rpm and you do not need to save USB ports (e.g. for another peripheral) then just stick to USB, otherwise consider FW or eSATA.