I have the Spin 360 GT which is the version they sell in Europe. It has the extra side protection like the i-Spin has, but the original insert and swivel button position. You can occasionally find that version in the UK and if you can, it's a nice compromise.
I don't find the button position is an issue in the slightest. We've had it in three different cars (Suzuki Swift, Hyundai i10, Peugeot 3008) and it's never been an issue because there's a gap underneath the lip of seat which touches the backrest of the vehicle seat, so you just slide your hand in from below/the side and it's really easy and free to reach. It would only be difficult to get to if you were trying to go down from above. I actually find some other spin seats have worse buttons because they are stiff, both of the Joie buttons are really easy to release.
However I've installed it in some cars for work and depending on the shape of the seats in your car, sometimes it really does press quite hard on the backrest of the seat and then it can be hard to access the button, and actually the spin isn't the easiest then either. But I get the impression this is the exception, rather than the norm. And it would be the same with spin or i-spin.
You do get LOADS of space in it relative to other seats and I do think you'd get a decent amount of extra time in it compared to a 105cm limited seat. However - on the 25th centile, is this going to make a huge amount of difference to you? 105cm is still likely to be about 4.5 - 5 years of age. And if you have a second child, you may well move your first into a 123 seat (for example Britax Advansafix) later which doesn't have the 105cm limit.
The other differences between the two are that the i-Spin has a memory foam headrest and the insert is a bit improved in it, the wedge isn't as thick and you get an extra recline level to make up for that. At 15 months on a low centile you will be borderline whether you want to use the wedge or not, but if you did decide to use it it's a bit more comfortable looking and doesn't make them sit on the buckle.
They can be wobbly in some cars. It's wobbly in 2 out of 3 we've had ours in. This isn't a safety issue but it looks a bit alarming.