I would say that they have there pro's and con's: -
Please note that this is just my experiences: -
Standard Group lessons - 5 - 9 people brilliant from a social perspective, we used to have such a great laugh and we all became really good friends by the end of it all and great for when your just starting out. You get encouragement and confidence from others, great from a morale perspective. On the downside once you get past a certain level your needs as a riders get side tracked and it says just a laugh and you don't really progress too much further.
Semi Private - 2 - 3 people in a lesson, more geared towards learning and perfecting what your doing wrong, get a little bit of company for those encouragement moments but it's more intensive.
Privates - I love them and now have these on my pony and don't bother with either of the above now, I need my instructor to concentrate soley on me and my pony and help us progress. I may try a semi private again in the future but for the time being it's just going to privates for me as I think you personally get far more out of it.
My perspective about lessons has somewhat changed since getting a share which is basically mine 100% of the time, as my lessons are about learning and improving, it's not the only time I get to ride so I have fun and relax during my hacks but schooling and lessons are intensive and about progress for me. Whereas previously when they were my only only source of riding they were about enjoyment, learning and social in that order.
Good luck with your group lesson and let us know how you get on