I prefer to use a 4 wire installation. It will remain in complince should a conductive path be established between both buildings sometime in the future.
I have rarely had to replace an underground feeder that was installed in a raceway. I have replaced or repaired quite a few direct buried cables, usually type USE, not UF. I think it usually comes down to workmanship (not nicking the insulation & making sure the first 6" of backfill is free of rocks). I'll personally stick with single conductors in a raceway.
Most folks will jump on the savings offered by Aluminum. Since I'm the one doing the pulling, I like it also. Workmanship is very important with Aluminum conductors. I haven't had any failures that I know about.
[This message has been edited by Tom (edited 03-26-2002).]
Few things are harder to put up with than the annoyance of a good example.