Do you need to move seats now? I had been planning to put together a little guide for you all but I wasn't expecting people to be looking at the next seat yet, I was aiming for more like Jan/Feb. If you wait until around 8/9 months you'll have more options. Wait until the baby seat is completely outgrown and you'll have more options still.
We have the Joie 360 Spin, it's OK. It's really great for the price, but there are some downsides to it, the inserts are quite bulky and confusing, which means most people end up taking them out too early (the i-Spin has slightly improved inserts). It doesn't have an anti-rebound bar to stabilise it, so it is quite rattly/wobbly in some cars which isn't a safety issue but can look really alarming as well as sounding annoying. It's been fine for us in 3/4 cars we've tried it in but in that one it was quite bad! The leg room for rear facing is pretty small. So it's fine to rear face until about 2 years old and after that, you need to either be really dedicated to the cause of rear facing otherwise most people end up turning their child around. Forward facing in a spin seat is not ideal because they are pushed higher and further towards the front of the car. Also I wasn't happy with the safety rating of it FF so I moved him to another FF seat.
Longevity wise DS2 would still fit into it by height, it's one of the taller seat shells on the market, but I weighed him the other day and he's 17.7kg, so it would be time for him to move out if he was still in it. He is only 3 years 4 months and I don't want him in just a seatbelt yet.
(There are actually six variations of the Joie spin seat now BTW, just to be extra confusing!)
Most swivel seats come with the base attached. The separate bases are generally only for those modular seats. It's a bit of a con as it seems to be suggested at the time that you're saving money by not needing to buy another base later, but that's a bit of a white lie as you normally don't need to!
OK so anyway, 3 big questions for choosing the next seat:
How big/small is your baby compared to average (growth centile line, or if you don't know, most recent weight/length/age)
How long do you want to rear face for? At 4 months you MUST stay rear facing but it's up to you when you turn around. Minimum 9kg/sitting for the weight based seats, minimum 76cm/15 months for the height based seats. Longer is better - a one year old is still pretty vulnerable to crash forces when forward facing. Rear facing is much safer and this is more important the younger the child. It's possible to rear face for the whole toddler seat bit and only move to forward facing when you move to a seatbelt (booster).
Do you need to move the seat between cars/do you have multiple cars you need to buy for?