I am considering buying an ExpressCard-based eSATA controller to access an external hard disk (1TB). However, this hard disk is also already available using simple USB 2 cable. Is it worth to buy such adapter to have better performance ? Is such eSATA controller's performance comparable to the one built-in my laptop ? Is there a PCMCIA-based eSATA controller that's better than its ExpressCard equivalent ? I plan on booting a Linux server from this hard disk.
eSATA is much faster than USB 2.0. USB 2.0 will top out at about 30-35 MBps while your hard drive will transfer 60 MBps+. See this comparison. It is exactly the same speed as internal SATA, meaning that yes its speed will be comparable to the internal hard drive.
Although I don't know that you will be able to boot of of either a PCMCIA or an ExpressCard controller though. When I used a PCMCIA eSATA card, the drive was not bootable because the PCMCIA controller drivers we're loaded until Windows started.