There are a TON of training services being offered by various training businesses right now. Board and train seems to be all the rage, day training is one option (my personal favorite!), walk and train, etc. etc. but the two mainstays that I think will always be offered will be private lessons and group classes. These services are easily by biggest sellers, and both have their pros and cons.
Private lessons can accelerate a dog's learning because they can learn in their own home with few distractions, and the dog won't need to generalize the new behaviors they learned in class to the home environment. They can also make scheduling more flexible, since it isn't a set class at a set time every week. If you have kids involved in school activities it can make it so that you don't have to miss an important topic due to a basketball game or concert. On the other hand, group classes can help to test behaviors around distractions, and allow young puppies to get that much needed socialization during playtime and playground exploration. Plus, if I'm being frank the price point for group classes doesn't hit the pocket book quite like private lessons will!
Now, should you enroll YOUR dog in private lessons or group lessons? If I'm being totally honest I think the vast majority of dogs learn MUCH better in private lessons; asking the dogs to learn new behaviors in a room full of other dogs (huge distractors!) in a brand new environment is automatically setting the dog up to fail, and it can be difficult to make real improvement in loose leash walking or come when called when there is limited space for each team (part of why I keep my class numbers so small!). There's enough time in private lessons to trouble shoot EXACTLY why you're having trouble with your dog, rather than trying to rush through an explanation in the five minutes between the end of one class and another's beginning. But not everyone is interested in private lessons, so if that's the case the question is: is your dog appropriate for group classes?
Unless you are looking at a specialty behavior class like a reactive dog or shy dog class, dogs enrolled in group classes shouldn't be overly reactive, aggressive, or fearful around dogs or people. This doesn't mean your dog can't bark at all, but if they are worked up to the point of now being able to cope they are not appropriate for classes. I know it seems counter-intuitive to take privates when you want your dog to "get used to" other dogs or new people, however there are skills you need in place before going into a group class, and it simply isn't fair to the other dogs in class to expose them to a potentially aggressive or fearful dog. It will make class far more stressful for everyone involved (including the owner!). You also shouldn't sign up for a group class if you only have one issue you need to work on such as polite leash walking; your money will be far better spent paying for one or two privates to work on that single problem behavior you're having rather than waiting for week 5 of a group class to finally get around to your biggest issue.
Lastly, if you're having any specific issues at home that ONLY manifest at home or that involve any kind of aggressive behavior (i.e. resource guarding, biting during nail trims, etc.) you are far better off signing up for private lessons. I simply cannot advise on modifying aggressive behavior to a group class student, and there isn't enough room in the group class syllabus to really dig into these intensive topics.
This post isn't meant to turn you off of group classes. Group classes are great for their affordability and for dogs that just need "the basics", or puppies that need that socialization time; additionally, if you're hoping to pass the CGC or CLASS test, or test your dog for therapy work a group class helps your dog learn to ignore other dogs while on leash and to focus with distractions, which are both critical skills they can't learn at home. Group classes also help to keep your dog used to new people and dogs in their lives; I've taken nosework and tricks classes with both of my dogs, and the new and novel experiences plus the exposure to different people and dogs helps to keep them well-rounded. They absolutely have their purpose and can be a helpful tool, and I think in an ideal world dog owners would be able to take advantage of both, but hopefully with these criteria in mind you will have a better idea of where your dog will fit best.