My wife and I have stayed at both Matagi (the original spelling - they added the "n" so people from the US could figure out how to say the name) and Vatulele multiple times.
Matagi is nice - the Douglas family run it both as a resort and their home, so everything is well looked after and the service remains of a high quality. The resort area is flat, but the island itself has areas with hiking trails that are quite steep & lengthy (purely optional). The diving is good, and they are now offering to arrange longer trips to Somosomo (full day trips). They are one of the few left that will dive Noel's Wall (named after Noel Douglas, now commonly known as Purple Wall) which is well beyond the old Forbes resort. This dive (at the right time w/the currents) is as good as almost anything in the Somosomo.
The only caution I would give is the traveling to get there. As mentioned previously, you fly into Nadi (usually arriving at night). Then you take a smaller plane (which will have steps up to the cabin) to Taveuni (with a stop first at Savusavu). From the Taveuni airport (really just a large shack) you take a short van ride (somewhat bumpy) to either an old dock or the beach for a 20 min boat ride to Matagi. One at least one end of this part of the trip you will likely have to wade into the water to get/in of the boat.
Vatulele is a 5+ star boutique resort that sits on it's own island south of Viti Levu. Because it is really the top of an underwater rock formation it (suprisingly) has several areas with nice walls, and each of the very small (like tiny) islands just away from Vatulele each provide some interesting formations to dive.
The diving here is actually very (very) good. We've seen many more pelagics here than at Matagi (including yearly sightings of very large 10ft+ Guitarfish lying in the sand). You also can see many more sharks here than in the north (but they are small, usually white tips and maybe a few Grey reefies). There are also more turtles, and you do get more of the critters you find elsewhere in Fiji (like scorpion leaf fish, etc.). What you don't get here is the volume of soft corals that you would see at Matagi (although smaller outcroppings exist).
From a diving perspective, neither Matagi nor Vatulele will compare to Namenalala or what you would see on the Nai'a (liveaboard) but I don't think either of these options would be great for your wife (Namenalala is a beautiful island w/amazing diving, but there's not a flat consecutive 3 ft on the island (except for the 7 secluded beaches), and there are over 100 stairs to/from the dock)
The reason to dive Vatulele is that you can do anything you want. The resort is set up to pamper to the wishes of the guests (because of the price you pay ~1Kk/night). Your wife would be extremely well taken care of here (essentially dive where you want, when you want for the most part). We've had the dive boat to ourselves on many days...
The food is outstanding (and you can order anything, on or off the menu), the wine cellar is the largest in Fiji, and the overall "experience" is what they are known for - you will not have a bad time here. This used to be the highest rated resort in Fiji, but over the last several years places like Wakaya (another place you should look into) have taken the lead (by being a little more over the top in what they offer honeymoon guests, which is the majority of the guests these types of resorts draw).
Only 1 warning: Vatulele gets a very mixed international (and somewhat well off crowd). This was always part of the appeal for us (liked meeting new people, for example Australia's #1 Elvis impersonator - he's made $M's from this - go figure). You will likely meet some folks that are a little "different" - everyone here has money, and they come to Vatulele because they don't want to think about anything for a week or two. Because dinner is served primarily at the long communal table you will spend time w/these folks (at least once a day) and some of these characters will likely emerge... it's one of the charms of the place (and one the things that the original owners tried to foster...it always felt like a very serious party was threatening to break out after each dinner). The resort is now run by the same folks who run Soneva Fushi (which we've also visited in the Maldives). This likely means there is a little more focus on privacy and "calmness" - but I don't know...