I often hear this being said but is it really true and what are the advantages of riding in either situation? Let's consider this with the Five Elements in mind.
Physiological improvements are dependant on time. Factors involved in improving aerobic fitness such as fat metabolism, cardiovascular adaptations (heart size, heart rate, cardiac output, etc:) require long durations to be spent training at the appropriate intensity. There are no short cuts for this one, sorry! Therefore the more time spent training at the lower intensities the better (to a point!), the turbo does not match a ride outside here.
Factors involved in anaerobic energy systems generally need a lower proportion of your training time than the aerobic systems. To generate a good high intensity session less time is needed and therefore fitting this into your standard turbo session is much more possible. You'll also find you can control the exact duration of both work and rest intervals without interruption from traffic, roundabouts and profile. Is this worth twice the time, probably not however, if it is the anaerobic energy systems you are trying to train, a turbo can certainly be an advantage.
Many people find using a turbo much tougher than being outside on the road. The lack of movement and change in gradient resulting in the more continuous pedalling is often seen as boring. However, if you want to train your ability to focus and be single minded, the turbo can do this for you. Is it worth twice the time outside? Well, this is much less measurable, and highly likely to be person specific.
Outside you are likely to pedal for about two-thirds of the time. Previously on an old school trainer, pedalling continuously throughout a turbo session would vary from what would occur outside. This clearly allows more time to practise the mechanics of cycling and a good pedal action whilst it also has the potential of reinforcing a bad pedalling technique. The ability to control the cadence and duration of intervals to the second is a huge advantage for a turbo. Twice the time? No, but probably around 50% more.
Cornering, descending, balance, group riding. All of these skills are noticeably absent on a turbo and to practise these skills you need to be outside.
Similar to the technical side of training, it's hard to replicate most group riding tactical decision making while on the turbo. However, if you want to work on your pacing skills for your harder efforts then this is possible inside and good for the same reasons as with the aerobic energy systems. So think about what your own race situation is here before choosing your preferred training method.
When considering other time commitments, if you are short on time then setting up a turbo doing your session and packing away can often be quicker than heading outside and getting the session done; is the time on the turbo worth twice the time outside? Probably not, but it is quite possible a time-saver especially if you are lucky enough to be able to keep your bike set up when you are not using it.
What about smart trainers?
With the increase of interactive smart trainers and associated programmes like Zwift, the difference in pedalling time between riding traditional turbo and outside can be adjusted and any longer sessions aimed at developing the aerobic system can replicate what is happening outside and any boredom can be reduced. The ability to make longer rides more realistic and to complete higher intensity sessions makes them an attractive option for many however, in response to our original question is it worth twice the time on the road, this obviously means you need to spend as much time on the turbo as you would outside to get similar effects in the technical element of pedalling.
Using a turbo is a great and at times, essential addition to training however, thinking in terms of a direct comparison between using it and a ride outside is not always helpful. Instead aim to see the benefits of both forms of training and choose which one you use with an understanding of what you are trying to get out of the session.