Why you want to learn is as important as where you study
While it's true that a lot of Poles speak English and in many cases a third language, to really experience the country you may want to learn Polish, or at least enough to get around.
Before even searching for a suitable course, decide why you want to learn. to become fully proficient can take a lot of time and unless you are interested in adding your name to the history of great Polish literature, a course that concentrates just on conversation may be the best for you.
Obviously recommendations from friends or on social media can help but prices can vary so it's definitely worth looking at a number of schools.
The size of the class can have a real bearing on how fast and how much you learn. While one-to-one classes are the fastest way to learn they are obviously more expensive. A good rule of thumb is the smaller the classes the more effective the course so look for details of what each school guarantees class size before committing.
Some schools use one style of learning ( i.e. Callan Method ) and others prefer a different style. All will swear by their chosen method but you are your own best guide as to how you learn. Take the time to investigate each schools style and choose the one that best suits you.
There are a lot of language schools in every Polish city so there is lots of competition. Be on the lookout for discounts offered on groupon etc. if you aren't locked in to particular dates. At the same time, invariably the cheaper it is the larger the class size so you will need to decide which is more important.
Does the school do field trips into town to allow you to practice your new polish skills in a real world environment ? What support do they give to slow learners ? Do they require you to buy text books from them at inflated prices or can you by them second hand?
The vast majority of Polish language schools are full of dedicated talented teachers who passionately want to promote their language so wherever you choose you are likely to have a great time and make some new friends in the process.