Logitech fan here. I've used an MX510, 518 and G5 and all perform well, are sturdy, reliable, tough, etc, in fact i still have all 3, and all 3 still work fine. Use the G5 at home, 518 at work. Have had issues with Razer products. Build quality, to me, seemed 'plasticky', and have had microswitches give out after relatively short periods of time.
imo important things are - build quality, ergonomics, drivers. DPI/speed/etc these days is ridiculous and the difference between 800dpi and 3200dpi is simply not going to make you a better gamer.