There are a number of important considerations when choosing backup media, of these tape capacity, transfer speed and longevity.

Care should be taken when considering tape capacity as manufacturers will often quote a compressed value. When comparing either transfer speed or capacity you should always ensure that it's the native values you use.

AIT
Currently, as of 4 January 2008, AIT-5 is the latest shipping version of this particular tape technology originated by Sony. It provides for native storage capacity upto 400GB at a transfer rate of 24MB/s.

DDS/DAT
This certainly is a story of rebounding branding. DAT was originally launched as a digital audio format, and was later utilised for data backup with the creation of DSS (Digital Data Storage). However the latest incarnations have returned to the DAT branding roots as the DAT72 and DAT160 formats. DAT72 utilises that same tape form-factor as earlier DDS models. However the DAT160 drives whilst maintaining read/write compatibility with earlier tapes actually use a slightly wider tape housed in a thicker case to achieve their higher 80GB capacity, at a rated transfer rate of 5MB/s.

DLT/LTO
DEC (Digital Equipment Corporation) designed the TK50 tape drives launched in 1984, which would later become DLT. The DLT tape technology was purchased by Quantum Corporation 1994 as part of a wider aquisition from DEC. Quantum maintain a more detailed history of DLT on their website. LTO, as it has the advantage that a number of vendors produce hardware, has gained substantial market share. Even Quantum, the effective owner of the DLT format, manufacturers LTO drives.

In terms of performance and capacity, DLT provides 800GB of storage at a transfer rate of 100-125MB/s, LTO provides 800GB of storage at a transfer rate of 120MB/s. However given comments by Quantum suggesting that they may entirely phase-out production of DLT hardware in favour of the LTO within a 2 years (from March 2007) LTO would appear the obvious choice, unless you have substantial investment in DLT already.

Which format is for me?
There is no simple answer to this question, it depends largely on your current and projected data backup requirements. If you have little data to backup, then one of the cheaper formats either AIT or DAT would make sense as not only are the drives cheaper but media costs are lower too. If you have larger volumes of data to backup then the choice becomes trickier. Multiple tape backups in lower cost drives may seem attractive, but are prone to error and require operator intervention. You could choose a DAT autoloader which can handle multiple tapes, but this pushes up the purchase price to a point where a higher capacity format may make better sense in the long run.

The only way to decide is to get an accurate measure of your storage requirements and it's rate of increase. Check the volume of data you currently backup this will give you a current volume. To obtain a rate of increase, which it's important to remember may vary due to a number of factors, either look back if you have sufficient backup history data or keep track of the backup size for a few weeks. I'd recommend having at least a month's worth of data before you even consider trying to use it to extrapolate future needs. I always try to plan for 18 months ahead, any more and you have no chance of being remotely correct, less and you'll be needing to buy replacement hardware in under a year.

If all else fails and you still can't find the budget for the hardware you need to backup all your companies data, maybe then is the time to check what you're actually backing-up. You'd be surprised how often the employees think you are responsible for backing up copies of all their family photos and their MP3 collection, solve that problem and you might find you've solved your budget problems too!