I recently subscribed to Joe Friel's newsletter. His most recent newsletter was relevant to this thread.

In 1999 the U. of Montana did a 2 year study using 14 amateur (but clearly serious) cross country skiers. The first year they performed an exercise regime of 12 hours per week of 'Polarized Training' with roughly 17% being at/above Lactate Threshold (or in this context at/above FTP I assume).

The 7 who responded "the best" to this training were called "high responders". The others were the "low responders".

In the second year the 'high responders" continued with the previous year's training regime. The 'low responders" ....

1) Dropped their total training volume by 22%
2) Upped their above LTHR training to 35% (which is larger by EITHER a percentage measurement or a absolute time measurement)

After the second year the 'high responders' results were unchanged. The 'low responders' results had caught up with the 'high responders' group. Of course this implies that, to the question, "which is best" the answer is "it depends".

Responses to training in cross-country skiers : Medicine & Science in Sports & Exercise is a link to the (somewhat dated) study.

Joe Friel - What?s Better for You: High Volume or High Intensity Training? is a link to this particular piece of Friel's blog.

Interesting (IMHO).

dave