You're probably better off not choosing carbonated liquid candy -- especially not doing so regularly and in significant quantities -- even if the sweetener happens to be non-caloric.
And the point where you've reduced your choices to regular soda vs. diet soda, you've already failed.
Along the same lines, there's no point in comparing the effectiveness of different forms of birth control because only abstinence is 100% effective.
Sometimes people want to eat and drink things because they taste good. There's nothing wrong with that, and we should be supportive of making marginal improvements rather than dismissing anything other than radical change.
In what way is it unhealthy? Aspartame has zero calories, and decades of studies showing its safety.
Put another way, if you want to avoid sugar (which is much less healthy than aspartame), besides water, there's tea, coffee, flavoring mixes like Crystal Light, and various prepared sugar free soft drinks. Many prefer variety over water. There's little scientific basis to believe drinking any of these things are bad. Drinking a mix of these is quite a bit healthier for an adult than chugging juice or sugared drinks - unless you're explicilty trying to carb-load. Milk is a healthy alternative but debatable due to its effects on some.
Not a direct answer, but noting that there's also a third choice, which is to not drink anything sweet. I'll chug a diet soda once in a while because what's the downside? But in light of new compelling evidence I might change that habit without too much fuss.
Diet. Artificial sweetener concerns aren't much different from "cell phones cause cancer" fears. Meta-analyses of decades of aspartame research shows it is safe.
Sugar, in liquid form, on the other hand - not so much.
C: Ice Mint Rooibos Tea, lemon water, lime water, green tea, sparkling water with a dash of lime (not "lime flavored bottled water, which usually contains sweeteners). If none of those satisfy your tongue, you might look into diet adjustments to re-calibrate your flavor response.
I can recalibrate my taste buds in all manner of ways, and sparkling water with a dash of lime will never even be in the same satisfaction category as an ice cold Coke Zero in a sunny day.
Had you mentioned a good lager, you'd have a contender, but there's another slew of health issues there.
In the end, living is slowly killing me. Let's just focus on the slow part and not fret about small risks.
And the point where you've reduced your choices to regular soda vs. diet soda, you've already failed.