I've had much better/stable wifi from add-in cards compared to USB ones.
I've actually had good luck with USB wifi dongles... when I run an extension cable (USB3.0 shielded) between the rear USB3.0 Type-A ports, through my desk cubby, to the top of my desk, and use a powered USB3.0 hub (that's tested to be stable), and then plug the USB2.0/3.0 wifi dongle into that. Enough power, plus enough height (on top of the desk), and away from behind or directly in front of the PC's metal chassis, gives me decent signal.
Then again, I'm in a relatively small 2-room apt.
I used to get cheap ($8-9 on sale), Rosewill N150 single-antenna PCI-E x1 wifi cards, to use for builds. Because they were cheap. But having only a single antenna, and being stuck behind the PC's chassis, performance was often very poor, like 1-3Mbit/sec sometimes, depending on router and placement. (That was back in the day, with N routers, and draft-N.) It could be as high as 25Mbit/sec though.
I generally used to use, for my personal systems, routers that had been flashed with DD-WRT or Tomato, and using them as wired to the PC, and then wifi over WDS or Client Bridge to my main router, in another room. (Early sort of "Mesh"-like system.) It had the added benefit of being compatible with Linux, assuming that your mobo's NIC was Linux-compatible, which the cheaper generic onboard RealTek NICs, as well as Intel NICs, almost certainly were.
Getting USB wifi dongles, that worked with XP, Windows Vista or 7, and Linux, was often a tall order. The router worked out way easier in the end, and I bought them all refurbished, for the most part, and they weren't much more expensive than some of the higher-end adapters were.