Resistance Bands

I believe they can do both so it depends on the resistance you choose...

For example the lighter bands (for you) doing 20 reps is similar in concept to a dumbell that you could do 20 reps for...

Conversely, if you choose a band you can only do six reps, it's similar to the six reps dumbbell..

Having lifted many years in the gym (10+  years)  and the past 2-3 years using resistance bands only, I'll say they are more similar than they are different.  Meaning poor form with dumbbells and poor form with bands is bad in both worlds..

If you want bands for toning, find the resistance to get you to 12-15-20 reps.  If you want to build muscle, find a band in the 4-6 range....

I'd recommend buying bands that are interchangeable in a set of handles....The bands that are pre-set can snap unexpectedly...

Pending the set up, they are also good for dry land swim exercises.  I have been doing a bunch since pool time is down. 

They can do both, but if your goal is building muscle you're better off using weights.  They do differ in that a 20 pound weight is going to be 20 pounds at the start and finish of your repetition, whereas bands get more resistant the deeper into the rep you get.  

Resistance bands are definitely more versatile and if you're trying to get home workouts in they're the much cheaper and better option.  You can use them while watching tv.  You can even make light resistant bands more resistant by using your feet to shorten the bands, holding excess with your foot, if that makes sense.