This is a constant query CESA is asked to provide a definitive answer on by language students (or their parents) whatever their current ability.
As with most questions black and white answers don't always help and generally the people you are asking have a vested interest in giving you one particular answer. So how do you decide the best approach for you?
For some students it's simply a question of budget. If you're young, funding the course yourself and keen to get the most linguistically for your money - group tuition is far and away the best solution. Far better to stay abroad for longer (giving you more time to soak up the culture, immerse yourself in the sounds, social etiquette and mannerisms of a language) than paying the steep bill that one to one tuition entails.
However some younger students, by this I mean the exam sitting teenager section of society, relish a little private tuition to boost self confidence, address any personal queries or weaknesses and give them a healthy push in the right direction prior to the latest academic hurdle in their life.
Private tuition is a god send to our adult students who either need the intensity and pace of learning to meet a very specific deadline, a presentation, new job abroad etc (which really needs a posting all of it's own to do the subject justice) or at the other end of the intensity scale our older students who plan on moving abroad when they retire, are learning a language as a hobby and simply wish to cover general conversational skills, without too much grammar at their own pace. Our older students love to take 10 or at the most 15 private lessons spread over a week, which leaves them free for plenty of indulgent lunches, sight seeing, exploring and relaxing.
Overall we'd say that the best plan is to spend time abroad - and lots of it - so you can practise your new language skills consistently. Trying to cram 40 private lessons into a week to maximise your language learning is rarely a good solution in the long run, unless your goal is clear and your stamina strong!