Nice call, Leokor.
I have both the Epson 1640Photo (work) and the Epson 2450 (home). They both do a great job and are very fast. I had used a Microtek E6 for *years* and had wanted to stick with SCSI for the speed factor (and CPU utilization) at work.
The new 2450 is, frankly, mind-blowing, both with regards to scanning quality and speed. DieHardware is right, with regards to total scanning workflow, the time scanning is only a small part, it's usually the transfer time into PS that has me tapping my foot. I have even used the 2450 for 35mm slides in a pinch, it's clearly not a dedicated slide scanner, but depending on your final output size it does a darn good job. As always, when choosing a scanner, you have to keep the final output in mind. Where are the images going to be viewed? Obviously the web requires much less scanning "power" than print. If you are going to print the images (even as small as 6 x 4), going cheap is a false economy because you will spend far more time making them look good from a so-so scan. This is somewhat analogous to the issues that people face when choosing a digital camera.
If speed is the issue, in my experience, SCSI or Firewire is the way to go. And franky, given my recent experience with the 2450....now my SCSI scanner seems slow 😉.
If you can afford it, go for the Epson 2450. It will serve you well on all accounts and comes with an *excellent* software bundle.
Good luck,
fraz