First, to ease your mind, I have been selling carseats for almost three years total. I worked at Babies R Us for almost that full three years, and recently switched to another baby store. So I have a ton of knowledge that you won't get from most reviews!
Second, always take review online, especially on baby products, with a grain of salt. A ton of people out there will give something a bad review if they don't know how to use something properly (which was probably the case for the Diono review you read).
Diono is actually one of my best suggestions, for tall kids and short kids alike. It's just an overall great seat. It has the same steel inside that a Britax has (which is what Britax such a hot seller). But it's considerably narrower and much easier to fit in a normal vehicle. As I'm sure you know, Britax carseats are MASSIVE. They're wide and bulky and take up a lot of space. You didn't mention in your OP if your baby still has to be rear-facing - if so, Diono is still the better option. Britax isn't the biggest out there when rear-facing, but it's not that easy to fit either. The way Diono sits is a little different than some other brands, and the end of the seat can be aimed in such a way that it won't hit the backs of the seats in front of it.
The main thing to consider with any size child is how high the back can go up. Some of the smaller carseat brands out there (including Britax) have headrests that move up higher. You want those to meet the top of your child's head no matter what. They should match up until your child is in a booster after 4 years old. A lot of carseat brands go up to 65 lbs, and even though they don't look like they do, they can do so safely. Just make sure you shop around and do a lot of research (not including customer reviews) before you buy.
Oh, one more thing to add: I don't suggest a 3-in-1 carseat under most circumstances. The ones that go from 5 lbs up to a booster. They're pointless. They're big for no reason. Yes they could save you money in the long run, but a booster later on down the line will only be about $20-$70 depending on the brand - not much of an expense.
Hopefully that helps!