Yea look WiFi is already SO MANY MANY times slower than USB, even on its best day it makes no real technological difference, other than POSSIBLY, if you believe that you are going to be on the very edge of your routers WiFi signal, that some internal WiFi cards have nice big antennas on a length of cord that allow you to place the antenna here & there until you get it just right - that said there are also USB dongles with antenna but without a USB extension cord they can be difficult to position...
So really whatever works best for you - if you are close enough to your WiFi source, you don't really care about loosing a USB port - and couldn't be bothered opening things up and messing around inside there is no problem with USB WiFi.
The only REAL issue could be if for example you have some REALLY super expensive latest revision WiFi router (you know the kind with about 7 antenna sticking out of it) that promises a Trillion Jiggabits per moment and you want to copy 300GB to and from a NAS on your router every night over WiFi, then you may very well want to invest in a high performance internal card that also has about 7 antenna sticking out of the back of your PC for full multichannel blibbedy blabbedy bloop but really that is just taking things way too far - apart from that just being completely ridiculous those super ultra multichannel blah blah routers are a pile of crap - there I said it!! They never deliver what they promise - although they do come close you need to be so damn physically close to it anyway to maintain any kind of real speed you could just plug a damn Ethernet cable in anyway and just do full Gigabit so why even bother with the WiFi?
If you are just looking for regular old b/g/n home Wifi for internet and things, just double check that if for example your router supports "n" that you actually get an "n" compliant USB dongle (or internal card or whatever) most routers will run their WiFi at the speed of the WEAKEST device on the network and an old model of device running mode "g" when everything else on the network is mode "n" compliant is far more often the cause of poor network performance than anything else.