Definitely, definitely, definitely a studio.
There are so many reasons for this I cannot even begin. And I've been doing yoga on and off for the past 5 years. And it's amazing esp. psychologically. It is also pretty hard at first.
There are too many reasons to list why a studio is better. It has to do with things from keeping better form, motivation, actually being in the flow and the moment for the full hour without distractions. But there's also something much more beneficial to doing it with a group of people. The only important thing is to ensure that you think of it as an individual practice and not something your doing in competition with others. Yoga is NOT competitive. And it's not about how well you do the postures or any posture but about synchronizing breath to movement.