As urban environmental problems such as climate change, air pollution, and the urban heat island effect intensify, the need for quantitative assessment of ecosystem services provided by urban green spaces is increasing. This study applied the i-Tree Eco model to compare ecosystem services provided by street trees in Suwon and Uijeongbu and to identify functional differences associated with urban structure and management characteristics. Attribute data, including species, diameter at breast height, total height, and crown width, were developed based on public street tree datasets. The dataset was integrated with meteorological and air pollution data to estimate carbon storage, carbon sequestration, oxygen production, air pollutant removal, stormwater runoff reduction, and their economic values. In addition, ecosystem services per tree and differences by land-use type (residential and commercial areas) were analyzed. The results showed that Suwon exhibited higher total ecosystem service provision, including carbon sequestration, oxygen production, air pollutant removal, and economic value, primarily due to its greater tree abundance and total leaf area. In contrast, Uijeongbu demonstrated higher ecosystem service efficiency per tree, as individual trees were relatively larger and had more developed crowns. Overall, ecosystem services provided by urban street trees are influenced not only by planting quantity but also by crown structure, growth condition, and management level, resulting in distinct inter-city differences in terms of total provision and per-tree efficiency. This study demonstrates the need to shift the paradigm of urban street tree management from quantitative expansion to function-oriented management based on an i-Tree Eco-based quantitative assessment.