We present a comparative experimental analysis of the thermal spectroscopic characteristics of a phosphosilicate (P)-based ytterbium-doped fiber (YDF) against an aluminosilicate (Al)-based YDF in the temperature range of 25 to $150^{circ}C$. We also characterize the fibers as gain media in a cladding-pumped amplifier configuration. While both fibers exhibit comparable trends in their thermal characteristics, there are noticeable distinctions in the fluorescence lifetime reduction rate and the spectral dependence of the transition cross-sections. The P- and Al-based YDFs present thermal lifetime reduction rates of $0.012%/^{circ}C$ and $0.026%/^{circ}C$, respectively. In particular, in the spectral region at ~940 nm, the absorption cross-section of the P-based YDF undergoes significantly less thermal change compared to that of the Al-YDF. In the cladding-pumped amplifier configuration operating at a total gain of 10 dB, the Al-based YDF generally performs betters than the P-based YDF in the temperature range of 25 to $75^{circ}C$. However, it is highlighted that in the high temperature range of over $75^{circ}C$, the latter shows a less gain reduction rate than the former, thereby yielding higher relative output power by 3.3% for a 1060-nm signal, for example.