Express cards can do 3GB/s. PCI's 1.5. But, that only makes a difference if the internal drive (or whatever your data source is) can go that fast.
The drive enclosure is esata/usb. Since it's hooked up to my notebook, I almost always connect via usb because I can hook it up to my hub and just have one long cable from the hub to my notebook (on my lap).
The esata cables I have are short and stiff. It's very difficult to use them unless the notebook will stay stationary on a desk. This is especially important because the express card slot releases the card when you push it in a little bit (like when trying to make sure the cable is firmly seated in the card after you just unintentionally gave it a tug!) and that's easy to do when the notebook is on your lap. If this happens when the drive is being written to, then you're in for some work cleaning up the mess that makes.
I ended up getting a T-EC2S from Radtech (the only one I found that actually stated that it worked with Linux), but the key is to get
a chipset that the kernel supports; the best supported is the Silicon Image SiI3124 or SiI3132, or any other chip that uses the sata_sil24 driver.