Re:
In terms of quality, I found the opposite to be true. My 20" acrylic has been noting but perfect for over 6 years now. My best friends 20" aluminum display is brighter (at full brightness) but the colors, even after calibration, do not seem as true. Neither of us run them at full brightness anyway. It's just too much!
Your mileage may vary, and every display is different, but I won't be afraid of the acrylic model just because it's older.
With the acrylic, it's true that you'll need a separate ADC to DVI adapter though, which will run you $80 to $90 more.
Lastly, consider the angle of viewing. The acrylic models adjusts by tilting away from you only, and they do not pivot side-to side easily. They have 2 "feet". The alu. models pivot more freely due to the middle-mounting point. They also free up much more desk space since they have a smaller footprint. They kind of float in the air more.
Good luck! Both would be great, I think.