it seems most people end up needing something to bridge the gap between the 0-4 and the high back booster seats.
This is because the 0-4 years seats vary in size and so do children. In general, if your child is within the 25th-91st centile (so, around 2/3rds of all children) these seats will last until somewhere between 3.5 and 4.5 years old. That means they might outgrow a little bit before their fourth birthday. In addition, a lot of these are starting to feel cramped by 3.5 or so, even though this is fine, parents often worry that their child is too big. If their shoulders can still fit under the headrest, and the harness is still on their shoulders, not dipping down below, and they are still within the stated weight and (to a lesser extent) height limits, it's still safe to use. Parents sometimes also need to hand the seat down to a younger sibling, depending on the age gap and the amount of time that you are able to use the infant carrier.
This all worked fine when the recommended age for booster seats was 3. But these days a lot of experts are saying 4 is the minimum recommended age. When I worked in fitting, our store policy was 3, so I test fitted a lot of 3 year olds, and IME there are basically no 3-3.5 year olds that sit well and look safe in a booster seat, and of the 3.5 - 4 year olds, some of them did well but a lot of them were immediately drawn to fiddle with the belt, wriggle around, slouch, complain, and generally I tended to recommend their parents look at a group 123 type seat which has the 5 point harness until 18kg and then you remove it.
I moved my older child (14 now) at 3 and it seemed fine. But I now have a 4y4m old and I am not ready to move him yet. He still tantrums, he still immediately fiddles with anything placed in his vicinity. He probably would sit nicely in a seatbelt but I tried it on him in the summer and it just looked too big and ridiculous, so I'll keep him in the harness until he outgrows, which will be any time as he's on the top settings now (was in the summer too).
So anyway.... long story short... it's probably not hugely necessary to your decision right now, but as you get to the age that your first child is outgrowing the baby seat, you may want to look at whether they are taller than average or a lot heavier than average, and if so, opt for a seat that is up to 6/7 rather than up to 4 for the next stage. OTOH, if you're likely to hand their second seat on to a sibling anyway, then the outgrown age won't matter so much, because you'll need 2 of the seats in this stage anyway. Yes, in theory you only need 3x seats, but if you have two (or more) children, then the stages are too long to pass them all down in sequence. You'll need at least 2 probably of the middle stage and/or high backed booster stage, long term.
May I ask, if I was to go for a standard, non spinning infant seat, and a spinning next stage seat, which two models would you recommend for these?
There's a lot of personal preference involved TBH but I personally used a Cybex Aton 5 (which I think is discontinued now, but Cybex have some newer "basic" models) and a Joie 360 Spin GT. I've been really happy with these two, but my approach is to find out what features REALLY matter to me and then find out how cheap I can go - it wouldn't be everyone's approach.
The only other suggestion I'd make is to look at the Avionaut infant seats, because they are newer to the market and less well known. They have a really good insert that positions the baby almost flat, very unusual in car seats, and the car seat itself is extremely lightweight. I think I probably wouldn't have paid the extra, I liked the price point on the Cybex, but they are worth looking at. And the Cybex Cloud Z is a lovely seat, just again, too expensive for me personally. I also really like the inserts in the Maxi Cosi Pebble seats, just a shame they are so short lasting. But as said earlier, that's not so much of a problem these days, as there is more of an overlap in the second stage seats, especially newer ones.
In terms of spin seats, the Joie is fantastic for the price, and you don't really need any extra features, but if you have a bit more budget I'd look at Britax Dualfix i-size, Recaro Salia and Besafe izi Turn because those have some nicer features that the original Joie doesn't have. People get a bit frustrated with the harness pads and the inserts on the Joie seat because they are bulkier than other seats, and it has very little leg room for rear facing older children, which a lot of people dislike, although some live with it. I am personally not a fan of the Cybex Sirona, most spin seats have a button to press to "release" the spin but with the Cybex, you push up a hollow between the child's legs, which is the same mechanism as the recline and it was forever getting stuck when I demoed it. I think it would be difficult with a larger child as well and it sometimes jolted quite abruptly which might wake up a sleeping baby/toddler. The seat is also short so it's outgrown sooner than other brands, although the harness is lovely and smooth to use - it's good if you have a child on the smaller end.