Google Voice is basically a phone number you get over the internet. You can receive calls on it, check your voicemail, forward calls to another phone, and call out too, including internationally. It is more like setting up a second phone line than a quick way to make one call abroad.
Talkala is simpler on purpose. You open it in your browser, type in the number you want to call, see what it costs per minute, and dial. There is no monthly calling plan just to place calls. You add credit when you need it.
The difference matters depending on what you are trying to do. If you already use Google Voice for your number and voicemail, using it for international calls too makes sense. But if you just need to call a bank, an airline, a doctor's office, or family abroad and you don't have any of that set up yet, Google Voice can feel like a lot of steps for one phone call.