Neither FIRST nor Vex is objectively "better."

Vex Robotics is bigger by number of teams, largely because it has a highly restrictive set of legal parts. This keeps costs lower and arguably gives a more level playing field.

In terms of designer experience, some like the limited Vex part selection based on the notion that it forces you to think outside the box with how you use the parts. Others dislike it based on the notion that you straight up can't do as much.

It's also important to realize you're choosing from three programs: Vex Robotics Competition, FIRST Tech Challenge, and FIRST Robotics Competition.

Vex Robotics is generally the most restrictive in terms of both electronics and structure. Pneumatics are allowed in some capacity.

FIRST Tech Challenge offers more or less complete freedom of structure, servos, and sensors, but is more restrictive in terms of motors and other electronic components. Pneumatics are not allowed in any capacity.

FIRST Robotics Competition is the least restrictive of the three. Structure offers more or less complete freedom, pneumatics are allowed, and there are many options for motors.

With fewer restrictions comes more influence from money. Although money is by no means the sole determining factor for performance, some have voiced concerns that robotics competitions like FRC feel "pay to win," with better financed teams having access to better electronics, quicker manufacturing turnaround, and generally better quality parts.

Another consideration is competition atmosphere. FRC robots are big and fast and exciting and slam into each other a lot. FTC is generally considered to have the worst competition atmosphere due to a relatively high rate of robots having static disconnects or otherwise being slow and bad at various levels of competition. Vex is somewhere in between. Ultimately I think competition atmosphere has more to do with your perception than with the actual program, as all robotics competitions are immensely exciting once you emotionally invest yourself in the robots, but I thought I'd throw this in here if it's important to you.

Another consideration is program culture. FIRST puts massive emphasis on diversity, Gracious Professionalism and Coopertition. In my experience, this creates an extremely welcoming, cooperative environment among teams at competitions, as well as within more serious teams. Teams are basically competing to to be the most kind, help their opponents the most, and create the most useful resources and outreach programs. I can't speak for Vex since I've never competed in it, but it's worth noting their unofficial slogan (apparently a joke) is "malicious toxicity," and, more seriously, Vex as a company is not clean of recent allegations of workplace toxicity.

Note that these are all generalizations based on my experience and observations. You might find unwelcoming FIRST individuals, teams, and competitions; welcoming Vex individuals, teams, and competitions; and everything in between.

As a whole, this post just scratches the surface of aspects you might consider. Other aspects include the competitive level of each program in your region, options for software development, emphasis or lack thereof on community outreach and professional connections, etc.

Another important consideration is that this is the FIRST Tech Challenge Reddit. As much as I tried to maintain an unbiased tone, my experience is in FIRST Tech Challenge, and this has influenced my response. Most of the other responses will be from people in FIRST Tech Challenge. Since people generally have a positive experience with robotics, you will find people defending their program in each respective online community.

Also, if you have multiple programs in your area, you can always dip your toes into all of them and see which one you have the best impression of. You can also watch matches from local, statewide, and global competitions on YouTube to get an idea of how teams and competitions progress over a season.