One important thing is your approach to your limited time. So you realize you don't have enough time to train. What is the best response? Well, you could do what the majority would do and think: “It’s not worth it, I’ll train tomorrow.” Or you could be positive and proactive and think: “How can I use this hour to get the maximum possible benefit? Maybe a one-hour time trial? Or an intensive interval session?”
From a strict point of time effectiveness, I would recommend you to mix your FTP workouts with VO2 max sessions. Why? In my view, there is a common myth that training at threshold power is the optimum way to train. In fact, there is nothing magical about training exactly at functional threshold power. This is because training a little above or below threshold power boosts performance in almost the same way.
I think athletes should train close to their VO2 max because it helps them improve BOTH their VO2 max and threshold power. A combination of VO2 max and threshold power training can benefit ALL riders and, done correctly, they will achieve fantastic results using either threshold or VO2 intervals - or possibly a perfect combination of both.
When performed at the right intensity, VO2 intervals can have a positive impact on your performance and they are also are extremely time effective. I believe that riders of all levels should include them in their sessions and that they should definitely become an integral part of training programs for elite and professional cyclists.
Elite and pro riders need training at (or very close to) VO2max to keep further progress. Thus, VO2 intervals are always included in the training programs I design for top athletes and I will also strongly recommend them to any ambitious athlete who wants to continue their progress.