I found the USB modems to be more reliable however they are definiately more expensive. Watching one 5 minute youtube video will drain a lot of your credit and I once ended up using 50LE (roughly £5) in a day.
The internet cables are generally a lot cheaper however you’re subject to the whimps of whoever is providing you with the connection. I was using my neighbours connection and paying him around 60LE (£6) a month but sometimes he would just unplug it if he was downloading something himself, which was quite frustrating at times.
Sometimes when I’ve gone for 2 months I just rely on the internet cafes as then I don’t get distracted from my studies and then I can go for an hour every 2 days and do everything I need to and it would cost me abour 2LE (20p) for an hour.
And Allah knows best