I´ve been to Malta(and Gozo) 2 times and to a few of the different Canary Islands....just so you know where I´m coming from...
Both will propably be warm enough for you (meaning you can dive them comfortably in wetsuits), Malta is the warmest of the two, specially in the summer.
Depending on which Canary Island you go to, marine life will vary but I think all will have "enough" for you to have stuff to look at (at least the more common islands like GC, FV and Lanzarote). There is an island that I believe is called Herrero (sp?) that is a nature-reserve that supposedly has a lot of marine life.
Malta propably has a bit less marine life but compensates, IMO, by having better visibility, caverns and wrecks that are shallow and relatively "harmless" even for less experienced divers. It could be a chance for you to "try out" som nice wrecks and caverns to see if its something you´d enjoy learning more about...
Almost all of Malta is shorediving(boattrips arent worth it in my experience) that is accessible by car(traffic is "italian rules" though so caution is advised) and so you and your wife could just rent a car and gear and then take things at your own pace which would be harder to do on most of the Canary Islands, something to think about...
Malta also have a few "topside" cultural atractions and, I would say, generally are less "touristy" than the Canaries.
I would vote for Malta because I enjoyed the diving and shore-life more than I´ve done on the larger Canary Islands but I´m sure someone will be along shortly to explain why it´s the other way around

That´s the beauty of forums like this one...Just ask if you have any more questions...