I'm a developer, and I already know from a software perspective there is a difference between a tool and an application ... so I'm not asking is there a pedantic difference in definition here. But I think the difference isn't obvious to those not expert in software.
So are these two tags each adding distinct value to the site, or are they being used in a redundant way? If they are redundant as used, which tag should be kept as the master tag during a merge?
Maybe a new tag like online-calculator would be better than online-tool to tag those one-off, simple, anonymous tools that don't persist or manage any personal data but only provide calculations or estimates from data entered on the spot? From a personal finance perspective, I think people go looking for calculators, but they don't go looking for "tools". Then, online-application would used be for services that do provide personalized, advanced features.
What do you think?