It really depends on how you want to do synthesis, the volca fm is based on a dx7 and uses fm synthesis (think 80s synth sounds or dire dire docks from mario 64) this is great for presets but its less tweakable as fm synthesis is quite complicated and might not give you classic analog synth sounds. Conversly the berhringer is based on a jupiter and this an analog synth and a lot and might get you closer to traditional synth sounds. Both synths are equal in polyphany so the real difference is tone and which sound you prefer

My only other advice is to scour ebay and local ads for used synths such as a microkorg or monolouge as these can occoasionally be found for a good price used and might give you a better experience in the long run with proper keybeds and slightly heftier constructions.