Depends on what you are using. There are functions only exclusive to the 5508 (such as OfficeExtend, etc.) that the 4400 won't do.
The 5508, being the newest-kid-on-the-block, is alot faster and can upgrade the APs more. It also has 8 1Gb SFP ports. Due to the extinction of the serial port of laptops, the 5508 supports USB console.
In my personal opinion, I'd go with the 5508.
