Many people place the decision to buy a particular data card model based on price (read free) but as many people know from cellphone selection not all device models are made equal and "free" comes at a price.
Even if cellular data cards don't have any fancy features to entice users here are a few selection criteria that will prove helpful for most users:
Availability of external antenna port (well most new modems have this anyway: - important if you need to add a high gain antenna for areas with low received signal strength (RSS). A cheap 5dBi antenna can do a lot! Even in areas with decent RSS one might want to put modem in a cellular router in a router enclosure (essentially an RF shield) and hence will need a antenna port to place an antenna outside the enclosure.
Availability of compatible router. Not all aircards have a compatible router more
Form factor. Cellular data cards available as Express cards, PCMCIA cards or USB cards. While USB cards might seem as the logical choice if one wants to use it on a laptop or desktop consider it's performance limitations pricipally due to USB slot power handling capability. Note also Express cards to date use intenal USB2.0 and NOT PCI-Express. Converters exist to adapt one type to another.
Cellular radio RF & baseband specifications. Just like a cellphone the better the internal device specifications the better the performance. This includes things such as receiver sensitivity, transmit power, power dissipation, MTBF (time before failure), etc. Some spec's are harder to quantize objectively. Usually (definitely not always) the more expensive the card the larger the BOM (bill of materials - how expensive the components were) and hence the higher the performance.
Availability of extra features such as GPS (commonly A-GPS), non-volatile memory for storage (Flash memory, Mini/Micro SD slots), etc.
International compatibility. Similar to cellphones this factor plays a role for those international business travelers or people planning an oversees vacation. Interestingly whereas GSM is very popular around the world CDMA2000 1x-EVDO is being deployed by many international carriers just as WCDMA UMTS/HSDPA (which is in the GSM evolution path).
