The two you've suggested are not quite the same, the Graco is based on the older, non i-size version of the Joie seat.
The differences are that the i-Spin has an extra recline position, a slightly thinner infant wedge - the inserts in the Graco are bulky in comparison.
The release button for the spin is on the side in the i-Spin vs under the feet in the Graco, can be tricky to reach if it's a tight fit in the car.
The i-Spin has memory foam in the headrest whereas the Graco just has ordinary foam. This has a comfort function but also a safety function as it absorbs more of a side impact. The i-Spin has a pop out thingy on the side to help with side impact protection.
There is an i-size version of the Graco but this is still based on the original design and just has a bit of extra side impact protection.
I would say that both the Joie and Graco are quite lacking in leg room as children get older. If you're planning to continue rear facing to a longer age, you might want to consider something with better leg room.
They both have good safety ratings (the non i-size Graco is not so good on side impact protection) so no worries there.
Considering the Joie is about £100-150 more than the Graco, I'd say the Graco is the better option out of these two. If you have £230-280 to spend which is what the Joie i-Spin 360 is priced at, it's worth comparing with the following, which I think are generally nicer seats than the Joie i-Spin and currently on offer around £200-230:
Cybex Sirona Gi i-size
Britax Dualfix M Plus i-size
Maxi Cosi Mica Eco
The Maxi Cosi might not get a great fit without the newborn insert. The Britax has a slightly higher weight limit than other spin seats, which is helpful.