Short answer: pick the gym closest to your daily route, tour it at the hour you would actually train, read the contract's cancellation terms before signing, and judge the floor by its members, not its equipment list.
Location beats everything. The research on habit is unambiguous: distance kills consistency. A gym on the way to work beats a nicer one twenty minutes out of your way, every time, for everyone.
Tour at your real hour. A gym at 2 pm and the same gym at 6 pm are different businesses. Visit when you would train and check the queue for the equipment you actually use.
Read the exit terms first. Month-to-month, annual contract, cancellation fees, freeze policies: the paperwork tells you how confident the gym is that you will want to stay. Clear, easy terms are a good sign about everything else.
Match the room to the goal. Powerlifting culture, class-driven studios, family recreation with childcare: Jacksonville has all of it, from big boxes to garage-sized independents, across /directory/?cat=Fitness+%26+Gyms in the /directory.
Then stack the deck. A training partner, a class time you protect, and a /member-deals check for member offers before you sign anything. The gym you choose matters less than the system that keeps you walking in.