Equipment
(modems) dongle vs usb stick?
Usb sticks are capable of recieving and transmitting the fastest data rates typically up to 7.2 mbps, however most mobile broadband providers (with the exception of Vodaphone) have maximum speeds of less than half this at present.
Usb dongles are adequate for the majority of networks at the present time unless your networks speed is in excess of 3.6mbps.
The only palpable beneifits of usb stick modems at present (assuming your network speed is less than 3.6 mbps) are "futureproofing" or erganomic (you just prefer the "look")
Which contract is best for me?
Fixed monthly data allowance
Where you pre buy a quanity of bandwidth between 1 and whatever number of gigabytes for a flat sum each month. Should you go over this amount you will usually be billed seprately for each megabyte over your allowance similar to a mobile phone contract. Sofftware is available for you to keep track of your usage.
Unlimited data allowance
As the title implies you are able to download as much bandwidth as you wish for a flat sum each month, however this is subject to the usual cavate of a "fair usage policy" which usually means there is a limit hiding in the small print and you need to compare this to the fixed allowance price plans to judge value.
Pay as you go
No fixed monthy bill just pay for what you use usually by the megabyte or gigabyte, top ups can be purchased from high street shops or online.
Contract length
In order to get the cheapest rates providers require a long term commitment on your part and with the exception of pay as you go contracts they are between one and two years. Therefore you need to take a long term view of your needs, for instance if you require mobile broadband solely for home use and there's currently no wired broadband available to your home. you wont need a two year contract if the local phone company plans to supply wired broadband in one years time.
Other considerations
Coverage
As discussed earlier check out who supplies coverage in the areas you live, work, travel, will you need to use mobile broadband overseas? What are the networks future plans for your area?
Speed
Do you need the fastest speeds? online gaming requires as fast a connection as possible as does video streaming, downloading will also be much faster but if you are in no hurry to get your downloads or only surf do you need it?
Equipment
All USB modems are not the same, some transfer data faster than others most have a top speed of 3.8 mbps so if you must have the fastest speeds (7.2 mbps) ensure your modems spec is capable of this.
The Future?
Consider future network plans for coverage and connection speeds in your area? you could have a situation where networks A and B cover your area, A is faster but more expensive, however B is much cheaper but plans to provide an equivalent speed in 3 months or networks AB and C currently serve your area but D plans to launch shortly and suits your needs best?
In these situations consider pay as you go as a stop gap