- in general way more flexible.
- USB 2.0 still dominates the market but will be replaced by 3.0 soon.
- USB 3.0 devices will be more common in the future
- allows for longer cable length
- your friends will have USB disks and if they have a eSATA disk it will have a USB plug as well.
- eSata probably slightly better for exclusive external disk (or disk array) usage
- eSata 3.0 is slightly faster than USB 3.0 on a bus level. eSATA 3.0 6Gb/s vs USB 3.0 5GB/s but those values are rather theoretical cause there are no hard disks you will be able to buy that support such speeds.
- should be considered if you work with large data on a external disk array that uses several physical hard drives to maximize throughput
Just to compare a computer hardware website has following in the section of 3.5" external disks as of Nov '11:
eSATA (153) SAS (7) USB 2.0 (378) USB 3.0 (156) FireWire 800 (84) FireWire (101)
USB 3.0 already is as on the same level as eSATA and is only going to get more popular.
The laptop with external USB 3.0 seems to be the better choice. eSata devices are more expensive and less common. The few advantages eSata has over USB 3.0 are no match to the wide range USB 3.0 has to offer.