Presently, a majority of studies concerning application of silicon photomultipliers (SiPMs) in positron emission tomography detectors are focused on scintillators containing lanthanum (LaBr 3 ) or lutetium (LSO, LYSO, LFS). However, the modules with the well known BGO in combination with SiPM light readout are also interesting due to unique features of SiPMs. In this work the two types of detectors, based on BGO and LSO scintillators are compared in terms of requirements for positron emission tomography scanners. The presented studies are performed with two Hamamatsu Multi Pixel Photon Counter (MPPC) arrays of 2×2 channels, with total area of 6×6 mm 2 and micro-pixel size of 25 μm (S10985-025C) and 50 μm (S10985-050C). The measurements of a number of photoelectrons, energy resolution at 511 keV and 22 Na time resolution are reported for various sizes of scintillators, including 3×3 mm 2 pixels and single crystals covering the whole MPPC active area. The paper is more focused on optimization of the system with BGO since its performance in combination with SiPM light readout is less known. The aim of this work is to show advantages of a SiPM based detector, especially in combination with BGO, in respect to the block detectors where classic photomultipliers are used.