The acidity in citrus fruits--particularly lemons--can cause damage to dental enamel if you eat them frequently and don't wash your mouth or brush your teeth immediately afterward.
Many soft drinks are also acidic, and the sugar in the full-sugar variety cause dental decay.
Sugary soft drinks are implicated in metabolic syndrome, which leads to Diabetes II. That's a very strong motive for steering clear of them.
Sugar-free soft drinks aren't healthy drinks, either. Both sugared and sugar-free soft drinks contain phosphorus, which decreases calcium absorption by the body and can lead to osteoporosis.
Water with a splash of non-sweetened fruit juice or tea with a bit of lemon probably aren't harmful, but it's a good idea to clean your mouth and teeth after drinking them. (Tea can stain your tooth enamel, also.)
However, I'm neither a dentist or a doctor. These are the experts you should ask if this is an important question (and answer) to you.