If it's in your budget, definitely go with LTO. They are some amazing tape drives, and very very fast. LTO drives also generally outpace the speed of IDE hard drives in a firewire or USB enclosure, thus allowing a quicker backup off a RAID device. Basically, LTO is similar to DLT in how it works, but it also adds a memory chip to the tape to store things like the catalog to speed up access. This is the same trick AIT uses to be faster then DAT(DDS), with AIT being very similar to the DAT tapes.
SDLT is an evolution of DLT but really doesn't offer any benefits over what DLT was beyond larger storage space. It is still generally more reliable then AIT or DAT though.