Unknown. The link speed (what is listed as 300Mbit or 600Mbit) is dependent on many thing. The biggest factor is the router you are connecting to. If you have a 300Mbit router, then the two WIFI options are the same. If that 600Mbit card is 300Mbit on 2.4Ghz and 300Mbit on 5Ghz, then they will be the same.
You would need to provide more info, router model, specific WIFI adapter models. Even with that info, it would be just a guess. The antenna of the internal card could be blocked by having the case between the router and the antenna. A USB adapter on the front of the case would work better. There is no absolute answer.