Hi M,
DW5530 and DW 5540 both are good WWAN cards. However, DW5540 is newer than DW5530
The suitable WWAN card for the system depends on various factors like region, service providers, services offered etc.
I would suggest you to contact the local sales team of your country to purchase the card. If you are residing in US you may contact Dell Sales at: 1-800-372-3355.
Please feel free to reply if you have any questions. I will be glad to assist.