In my limited experience, USB seems to be better - not least because you can stick it on a USB cable and blu-tack it to a wall above the computer, rather than having it hidden down behing an isolating and noisy computer case.
Using Safecom products (ie cheapie-stuff), I easily get 30 metres (& think it did OK at double this) between USB adapters (802.11b), whilst the PCI will work at that range, but shows a slight drop in performance. 802.11g will give a stronger signal, but I'm a skinflint, & won't pay the extra. I don't think you necessarily get a better or stronger signal from the "posh" brands. So, mine too came from Ebuyer, product code 48444 . Unlikely possibility - how powerful is your computer? It might just have a problem maintaining the connection if it is old