Originally Posted by sivat

Like the other 2 said, square taper is better for single-speed and fixed. Square taper bbs are available in just about any length from 103mm to 120mm. Somewhere in there you will find a length that will put you within 2mm or so of a perfect chainline. Since splined bbs are available in only 1 or 2 (depending on the kind) lengths, you just have to hope the chainline will be close.

As for which works better in general, without looking at adjustability, square taper has been around for years and is only recently being replaced by external bearings. It works well, lasts a long time, and is easy to install. There have been complaints that some of the spline designs tend to loosen over time.