I think you should first define "Active". For some people active means they're hiking and running marathons anytime they're not at work. For others "active" might just mean not couch potatoes.
Beagle pros: Affectionate, typically very friendly and tolerant, good natured animals, often good around children, adorable, medium to low energy, but they enjoy activity and can excel in sports like agility, very food motivated in training so it's easy to find a reward they desire.
Beagle cons: Stubborn, they'll eat anything and everything that crosses their path--even if it's a small live animal, some are resource guarders, they follow their noses and are prone to wander, they have a very dandery coat and shed a lot, so can affect people with allergies more than other breeds, they howl/bay, some are difficult to housetrain.
JRT pros: Very intelligent, very trainable and love to learn new things, the most high energy small dog I can think of, excellent for dog sports and doing activities with their owners, great ratters (I don't think many people seek them out for this anymore, except for maybe some farms, but I think it's a pro!)
JRT cons: Usually hard personalities, I don't want to say "dominant" but they can be intense and have a pushy attitude, can be snappy, very high prey drive and can also be reactive to other dogs, tend to bark a lot, their intelligence and high energy can be a con because if they are not properly exercised and mentally stimulated they can be destructive.
It might be clear I favor Beagles but these were the main things I could think of. I think if you're a bit more clear on what you want it could help you make a better choice. I will say that I personally favor Beagles because of their sweet personalities, and lower energy. JRTs are great if you want a dog for a sport, TV (which is why there's so many in show business), high levels of training, or to do something with them. They're kind of the Border Collies of small dogs. I am not interested in dogs like that, and I have seen quite a few that are snappy and reactive to people and other dogs.
Also I have to add that these are two of the most common breeds I see in shelters if you're considering adoption. I actually know someone who runs a shelter who has half a dozen Beagles she's looking to rescue out to other states if you're interested. Plus there's the Beagle Freedom Project that always has Beagles that were rescued from cruel Laboratory experiments.