A friend is moving to DC. I currently have a SmarTrip card. And, I recommended that they do the same. However, they asked me about the CharmCard (which apparently costs less).
WMATA SmarTrip
A permanent, rechargeable farecard that can hold up to $300 in value to pay for Metrorail and Metrobus fares and for parking fees at Metro operated parking facilities. The card costs $5 and comes with $25 in value already on the card for a total of $30. You can add value to the card at any Metrorail station or on any Metrobus.
MTA CharmCard
A permanent, rechargeable farecard that can hold up to $200 in value to pay for rail and bus fares and for parking fees at Metro operated parking facilities. The card costs $2.50 and comes with $7.50 in value already on the card for a total of $10. You can add value to the card at MTA ticket vending machines, MTA bus farebox, Metrorail stations, on Metrobus or buses at participating transit agencies.
It appears that if you primarily use the WMATA you can save some money on the card cost by using the MTA's CharmCard instead. Any caveats to using the CharmCard vs the SmarTrip card?