There are three other (perhaps more) considerations:
1. String height - Some guitars have the strings adjusted so low, its almost impossible to play without hitting the frets.
2. Muting - When playing slide, its important to mute strings. This help to keep noise at a minimum. Hard to explain in one paragraph, but check out Anders' tutorial for ZZ Top's Tush. Observe what the other fingers on the left hand are doing.
http://www.guitartricks.com/lesson.php?input=18852&s_id=1449
3. Open Tunings - What beginning slide players often don't realize, is that its a lot easier to play on a guitar which is set to an open tuning. My faves are D and G.
Dropped D:
----------
6 5 4 3 2 1
D A D G B E
Example: Hats off to Roy Harper, Led Zeppelin
Dropped G:
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6 5 4 3 2 1
D G D G B D
Example: Move it on over, George Thorogood
Hope this helps,
Izzy