I'm actually waiting on the A to make an appearance here in America because I'm going to buy a couple more of them. The original intent was to use a Model A anyway on account of $10 being like 7% of the overall cost of the project!
You already touched on the ethernet port. That would be the biggest thing I would miss. I have found the ethernet port to be very valuable when trying to get the wifi setup. I suppose you could do the SD card setup on the model b and then pop it in the model a. ???
/etc/config/network config interface 'wwan' option proto 'dhcp'
/etc/config/wireless option disabled '0' config wifi-iface option network 'wwan' option ssid 'yournetwork' option mode 'sta' option key 'password'