I have not had problems with different LTO units, there are basically only 4 manufacturers, the other brands are rebranded units from one of those four (Quantum, IBM, Tandberg, and HP). We have a Dell unit, which is really an IBM unit.
I would definitely buy LTO-4 instead of LTO-3 now, though, if you can afford it. You can still use (the slightly cheaper per GB) LTO-3 tapes in them, and you're much more future-proofed, and have larger capacity per tape and higher transfer speeds with LTO-4.



