I would agree with everything that jaxsprat stated except for the “little cheaper” part. Depending on which Ethernet cable(s) you have to get – it could be a lot cheaper! Example: for your two units (998 and 858) all you need is one Ethernet Connection cable. If you need a 20 foot cable it will run you about $60 versus about $130 for an InterLink.
AS EC 20E

AS INTERLINK :: Humminbird