The optimal selection of an interposer substrate is important in 2.5D systems, because its physical, material and electrical characteristics govern the overall system performance, reliability and cost. Several materials have been proposed that offer various tradeoffs including silicon, organic, glass and etc. In this paper, we conduct a quantitative comparison between two 2.5D IC designs based on silicon vs. liquid crystal polymer (LCP) interposer technologies in the overall system level for the first time. We also investigate tradeoffs in power, performance and area (PPA), signal integrity (SI) and power integrity (PI) depending on the interposer technologies. Through our flow, we generate a large-scale benchmark architecture with commercial-grade GDS layouts of interposer and chiplets using two different interposer substrates. Then, we model transmission lines and power delivery network (PDN) of each 2.5D IC design. Finally, we perform PPA analysis, SI and PI on both 2.5D IC designs to observe the quantitative tradeoffs between two designs. Our experiment shows that silicon interposer-based design has 10.46% less power, 0.25× smaller area and 0.57× shorter average wirelength compared to LCP interposer-based design. However, LCP-based design has 0.59× smaller PDN DC impedance and 0.75× shorter worst delay of interposer wire while maintaining the power delivery efficiency. Lastly, our cost analysis of 2.5D IC design indicates that the overall cost of organic LCP technology, if both the chiplets and their interposer costs are combined, is 2.69× higher than the silicon even the cost of LCP interposer is 1.91% of silicon interposer. This indicates that LCP technology is prohibitive unless the interconnect and bump dimensions are dramatically reduced.