There is quite a big difference between these two seats! You would be better off comparing the Joie Bold with the Britax Advansafix IMO.
The Evolva is seatbelt fitted only, with "isoflex" ie the isofix isn't the actual fitting method, it just helps hold the seat in place when it's in booster mode. Seatbelt fitting is good if that's the only option you have, but if you have the option to use a top tether installation, it's much safer and better to do so with a forward facing seat.
The weight limit of the Evolva harness is 18kg and it will expand about to the size of a child just outgrowing age 3-4 clothing. After this you must remove the harness in order to raise the headrest and use the seat only as a booster. So depending on the size of your toddler, this might push you into booster use before you necessarily want to use it.
The Bold OTOH has a harness weight limit of 25kg and it expands to the full height of the seat which means you should definitely get to that 25kg weight limit (or as far as you like towards it until you feel your child is safe in a booster seat.) This means for an average child you can keep them in the harness up to about 6-7 years, ifyou want to. For a child who is small for their age possibly older than this, for a child who is large for their age, it will probably only last until about age 5, but this is much better than a standard 123 seat with 18kg harness limit, which larger than average children will outgrow at age 3 or sometimes more like 2.5.
The fitting of the Bold is a bit more complicated. You must use isofix, top tether and seatbelt all at the same time. This means it's a very safe and secure install, but it can be a bit complicated to do.
I'd just add two more points about 123 seats in general - are you aware these type of seats are considered a bit of a compromise in terms of safety? Generally it's better to get a separate Group 1 harnessed seat, and then a high backed booster later. This is because the requirements of the two kinds of seat are quite different and trying to meet both in one seat often results in safety compromises. Top tether and isofix does go a good way to relieving this, but since you can get a very good quality high backed booster for about £30-40, it might be a false economy. I would say the Joie Bold is an exception to this (because of its unique harness weight capacity) and the other exception would be a child fairly close to the end of the Group 1 weight category for whom purchasing a standalone Group 1 seat feels a waste.
Secondly, it is worth noting that it's possible to buy car seats in groups 1 and 2 which face against the direction of travel, which is a much safer alternative for young children if they can tolerate it. Particularly if your child is near the beginning of the Group 1 weight class/just moving out of their baby seat, I'd highly recommend this.