Take the onboard credit and buy real insurance on your own. The free insurance these places offer is worth just about exactly what you pay for it.
Your agent did not have a choice in all probability on the coupon credit. That's different from a price drop which can be applied to an existing booking. Many of the coupons and other point of sale marketing things are to spur new bookings and say right on them "*For new bookings only, may not be applied to existing bookings".
I don't know anything about the two companies you listed. Be sure they don't have hidden booking charges or service charges for having to call them after you book. Some of the online companies have online booking only rates. That means you have to put your CC in the computer and if you need anything and have to call and talk to someone there are fees. As with anything else there are good and bad companies and good and bad agents. Hopefully someone on here will know more about them and be able to advise you.