Here is what I see as the main features of each:
Mailstation has no backlit display, but it has a 33.6kbps fax modem (but no mention of a fax application), it also has a 20,000 word spell checker (which is really slow and non-expandable). Printer support (most common printers), 5 separate email accounts, address book holds 1000 addresses, and it has 400k of storage. Max message size is 8k.
Mailbug does have a backlit display, but only has a 2400bps modem, only supports 1 email account, 120k of storage. Address book only supports 100 addresses, no printer option, but it does have Caller ID. Max message size is 16k.
Looks like Mailstation is the leader in features, now if only it had a backlight (and you didn't need to subscribe to their service!). IMAP4 support would make it really rock.