Non-adjustables are easier to work with because the weight is compacted rather than spread out. I curl about 30 pounds, doing 10 reps and 3 sets. I have an adjustable dumbell at my house that I put up to 30 and I can barely get the weight up. It's the same reason why it'd be preferable to put 2 45s on a bar, rather than 2 35s and 2 10s. The closer the weight is to your hands, the easier it's probably going to be to lift.
