We present a comparative far-infrared reflection spectroscopy study of phonons, phase transitions, spin-phonon, and electron-phonon interactions in isostructural multiferroic iron borates of gadolinium and terbium. The behavior of phonon modes registered in a wide temperature range is consistent with a weak first-order structural phase transition [Ts=143 for GdFe3(BO3)4 and 200 K for TbFe3(BO3)4] from a high-symmetry high-temperature R32 structure into a low-symmetry low-temperature P3121 one. The temperature dependences of frequencies, oscillator strengths, and damping constants of some low-frequency modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon mode behavior in both compounds at the temperature of an antiferromagnetic ordering [TN=32K for GdFe3(BO3)4 and 40 K for TbFe3(BO3)4] evidence the spin-phonon interaction. In the energy range of phonons, GdFe3(BO3)4 has no electronic levels, but TbFe3(BO3)4 possesses several. We observe an onset of new bands in the excitation spectrum of TbFe3(BO3)4 due to a resonance interaction between a lattice phonon and 4f electronic crystal-field (CF) excitations of Tb3+. This interaction causes delocalization of the CF excitations, their Davydov splitting, and formation of coupled electron-phonon modes.