Classical replica-exchange molecular dynamics simulations are performed to study structure, dynamics and thermostability of the 14-mer RNA hairpins uCACGg and cUUCGg. Despite of the different sequence and closing base pair of the two systems, recent NMR studies have shown that the tetraloop CACG is strikingly similar in overall geometry and hydrogen bonding to the canonical UUCG tetraloop. On the other hand, the two systems differ significantly in their functionality and thermostability. The simulations confirm the structural similarities of the two RNA hairpins at room temperature but also reveal that the UUCG loop is more flexible than the CACG loop. Concerning the functionality, the CACG loop shows a stronger attitude to donate hydrogens than the UUCG loop, although their global solvent accessible surface is quite similar. The simulations qualitatively reproduce the experimentally found difference in melting temperatures (20 K). In the case of the uCACGg hairpin, the thermal unfolding occurs cooperatively in an all-or-none fashion, while the cUUCGg hairpin shows less cooperativity but exhibits intermediate states during the unfolding process.