The difference between contract mobile phone deals and those that are paid on the usage is simple: the former will come with a set amount of usage for a set price, and if this is exceeded then extra charges will be incurred.
In terms of business usage, you may wish to investigate just what is included in the contract mobile phone deals that you look at, for there can be very significant differences indeed. For example, your users will need to be able to access and send emails regularly, so a deal that includes unlimited email usage may be worthwhile. Furthermore, they will most likely be communicating by voice, rather than by text, hence the greater number of minutes allowed in the mobile phone deal investigated is the better option. There are a great number of handsets on the market these days, and many of these are aimed at specific types of user: the business user, for instance, is not going to be interested in contract mobile phone deals that supply handsets geared towards entertainment, so it is worth understanding exactly what each different deal entails.
Look for a handset that is clearly business orientated – one with email and internet – and then find the best deal you can from the many available.