An LTO-3 will give you greater throughput, but an LTO-1 autoloader will give you greater data capacity.
The 3581 will backup 700-1400Gb of data across 7 tapes at a speed of 40-80Mb/sec. It will take about 1.4 hours per 100Gb of data – not including mount times.
An LTO-3 will backup 400-800 Gb of data on one tape at a speed of 80-160Mb/sec. It will take about 0.3 hour per 100Gb of data.
If you are under 400Gb of data, stick with the single LTO-3. If you have more data, either use the 3581 with an LTO1 or upgrade it to LTO3.
You should ONLY clean a LTO drive when prompted.