What is the Best Substitute for Soda/Is What I'm Drinking Okay?

I cut out regular soda about three weeks ago. In its place I've been drinking diet soda (mainly Coke Zero and diet Dr. Pepper). I know aspartame is bad for you, but I'm finding it difficult to drink just water.

I bought some Mio yesterday, and it's surprisingly good for zero calories. I've also been drinking a decent amount of Monster Rehab Orangeade. The caloric and sugar content are low (an entire can has 20 calories and 6 grams of sugar), but I'm concerned about the high amounts of caffeine and stimulants that are in it. Will they adversely affect my metabolism or have any other negative effects?