Neither option is better than the other, as which is best will be down to the quality of the tutor you choose.
We went down the road of both 1-2-1 as well as group tutoring. Can honestly say that THE most important thing to consider is the quality of the tutor.
If you can find a very good tutor, by default other issues such as size should already be well managed. We trialled sizes from 1 to 30+, after a very painful period of trial and error, we settled on the best option, which was only down to the tutor, and to our surprise was not 1-2-1.
The tutor we chose teaches in a small group, our son was in a group of 3 and he loved the buddy system that was set up, all 3 of his ‘buddies’ were aiming for the same school and all will be going together this year.
Problem is you will only know if you have a good tutor if you are armed with the knowledge of what to expect and what is considered to be ‘good’. We avoided any tutors employing assistants and group sizes more than 4, but you cannot beat personal recommendation.