Some of the main differences are detailed here. The QSR and S4+ are both excellent wind controller modules. Importantly, they both respond very fast to wind controller data and have a very smooth legato. The Alesis NanoSynth IS also capable of the smooth legato like the QSR and S4+. Please note that the older S4 model (non-PLUS model) does not have the super smooth legato capabilities of the three newer models. It isn’t capable of triggering new legato notes at the sample loop point but it still responds quickly and sounds good. One of these units layered with a TURBO VL70-m is great and highly recommended. The biggest drawback with these units is that their filter is not terribly steep and sounds thin.