You'll probably get a lot of different answers on this on, but here's mine...
Scale length is measured from the nut to the bridge, at the point where the string actually makes contact...where metal (string) touches metal (nut and bridge). Or, it could be touching bone, depending on what material the nut and bridge are made of.
Shorter scales are better for the slant bar technique. The "standard" is 22.5 inches. I put standard in quotes, because you'll find 22" scales, and I have a Harmony lap steel from the 40's with a 23" scale.
Longer scale usually refers to 24.5". But or course, there are 24" and 25" scales out there, too. As a general rule of thumb, the longer the scale, the better the sustain, and in most cases, clearer harmonics.
Fender made a 26" scale, which is popular with some players. Duke Ching plays one of these. It has a really sweet tone on the higher frets, where a short scale would start to wimp out in tone. They are quite rare, but any multi-neck Fender seems to be rare these days.
I've never seen a 27", but I'm sure there are plenty out there. The late Claude Brownell, who played in the Eharp tuning, used an extra long scale, which is the longest I've seen. I don't remember the exact dimension, but I know it was at least 31", maybe more. If someone out there knows for sure, please let me know.
An acoustic resonator is always longer scale, because since amplitude is the issue, a short scale just wouldn't cut the mustard (since there's no electronics to pipe the sound through an amp).
Which is best? It all depends upon who you ask. Each has their strengths and limitations. Personally, I have short scale instruments. As a student of Jerry Byrd and his course materials, a short scale is recommmended for optimal bar slants on the first few frets.
Now, if you really want to go crazy, combine your scale length dilemma with various string spacing options and a bunch of different tunings!
Sooo, which is best? Just pick something and go with it. JB once told me, "...it doesn't matter the guitar if you can't play. A terrible player will sound terrible on any guitar."
Hope this helps,
<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Jeff Strouse on 14 September 2003 at 02:32 PM.]</p></FONT>