Yes it can be a bit tricky to differentiate between very similar models.
Generally they will have a top of the line luxury one with all the safety bells and whistles, then they'll have a bare bones basic one which is cheaper, and the one(s) in the middle will be some kind of compromise between the two - sometimes comfort wise, sometimes safety feature wise, sometimes both. When you have extra models that don't appear directly on the brand website, they are probably last year's models, which is fine.
At 98cm and 15kg and on the lower end of three, I would strongly recommend a harnessed seat, it will have quite a significant benefit for him. Britax Evolvafix or Advansafix Pro, or Maxi Cosi Titan i-size (the Pro = better safety, Plus = better comfort, but just the i-size is fine in safety terms) are the ones I'd recommend, because they have good ratings both as harnessed seats and high backed boosters. If you can check them in store before you buy, you'll be able to see if you have more torso growth, my guess is the Britax will have more space (I have one and it lasted my son up to 109cm standing height - I can measure the max harness height tomorrow.) You're right you won't get a huge amount of extra space, but it does sound like the Mica is fairly short in the shell - there seem to be a spate of MN DC outgrowing them at the moment! If you can get 9-12 months more out of a harness it will be worth it I reckon, better if you can get even longer than this! Ideally you want children to be as close to 18kg/105cm as possible before changing to a booster - it's not always possible to get over, but over is ideal.
If these aren't big enough, you could look at Joie Bold R or Cozy n Safe Hudson as these both have harnesses which go up to 25kg and aren't so limited in height - but I would try the Britax first if you can since the safety is better than the Joie Bold and probably better than the Cozy n Safe. But if you do need the extra height in the harness, those two are an option. Don't bother with their new i-size versions as these are limited to 105cm (so you might as well get the Britax or Maxi Cosi).
I am not sure how tall the harness goes on the Maxi Cosi Titan i-size. The previous one, the Titan, had a short shell/harness and that was the same generation as the Mica that you have. I had no end of trouble fitting 3yos in this - the harness was SO short I stopped bothering with it after a while. But the newer Maxi Cosi Titan range (i-size, especially i-size pro/plus) look like they are bigger. I just haven't seen them in person to confirm.
Cybex Solution G is the mid range Cybex model while Solution T is the top range. The T one expands out widthways as the child grows whereas the G doesn't and I think the fabrics are probably more premium on the T. Cybex don't do a harnessed seat which changes to booster, only the impact shields.
People like Joie because they do a really great job of walking the line between quality, features and price. Nuna is the same company, and their high backed boosters have done really well in crash testing, but their harness + booster seat didn't get a very good rating, so would skip that one.