I am not 100% sure about this but I saw what looked like the 540 at EDNY on an outside stand, in sunlight, and was almost unreadable. It may have been one of their MFD products however, but it's worth checking out.
Aspen is cheaper, sure, but everybody (everybody) I personally know who has bought the EFD-1000 has had problems. One pilot is on his 4th unit in 3 years. Usually it is the sensor that packs up, but not always. Aspen are very responsive under warranty and in some cases outside the warranty but then they don't pay for the labour.