It all depends on what you are using it for.
If its general surfing 3gb is sufficent.
If your intending to stream or DL large amounts of Data i would go for the 5gb. ( however that is expensive if you look round you can get unlimited data for that on other networks)
Its also worth noting that the data is already compressed when using these devices so you get more than you would think as far as web pages are concerned etc.
These devices are really good and can run at up to 2meg+ depending on coverage versus the old data cards at 384kps.
Also they are usually plug and play so no software needed
Is is an 18 or 24 month contract?