Since Sprint & Verizon are generally considered the leaders in this technology, we will limit the discussion to these two choices. As far as monthly pricing goes they are exactly the same, $59.99 a month. However, there is a very important distinction. Sprint offers truly Unlimited service, where Verizon has a 5GB limit per month. For the casual user this is plenty of data, but for a power user, Sprint clearly is the better choice here. The other factor is coverage. Both carriers are going back and forth with their claims of "more coverage". The best thing to do is check the coverage maps and see who has the most Rev A coverage in the areas you need service. Rev A is the fastest service currently available, so go with the carrier that covers you the best, but if it seems equal, Sprint would be the better choice since you do not have any usage restrictions.
Now that we have picked a carrier, it is time to decide which card to get. This involves several steps depending on what you plan to use the card for.
If using the card only in your current laptop, then determine what kind of slot your computer has. There are 3 types of slots now in use.
1) PCMCIA or Type ll Card Slot.
This slot is the size of a Credit Card and has been around for over 5 years. Most older laptops have this slot. The advantage of using this type of card is that it keeps most of the card inside the laptop, with just about an inch of the card sticking out. You can install this card once and just leave it in the laptop. These are typically the least expensive option.
2) ExpressCard Slot.
This is the newer type of card slot that is about half the width of the PCMCIA card. Most of the newer laptops will have this slot. Once again, the advantage here is that most of the card is inside the computer and you can just leave it there without risk of it getting bumped or loosing it. ExpressCards are usually about $100 more than a PCMCIA card.
3) USB Card Slot.
Every modern computer has a USB slot. This is the main advantage of a USB Card - it will work with a new or old laptop, and even desktop computers. If you have an older laptop with a PCMCIA slot, but know you will be upgrading laptops in the next 6-18 months, then get a USB card so that you won't limit your selection of a new laptop. The one slight downside to a USB card is that most of it is external to the computer, making it a bit more to keep track of when transporting your laptop. USB cards are usually priced in between the two cards above.
Other factors to consider:
Mobile Routers.
If you want to use a Mobile Router to share your connection, currently you will need to stick with either the PCMCIA cards like the Novaltel S720, Pantech Px500, or a compatible USB card. The ExpressCards wll not work in the popular KR1 router, but will in the Top Global MB6000. However, an adapter is needed to make this work.
GPS Functionality.
To take advantage of some neat and free GPS functions like finding the nearest restaurant, gas station, mapping your location, and even real time traffic updates, go with a Sierra Wireless Card from Sprint. The Sprint Connection Manager for these cards allows you to use this service at no extra charge.