I haven't used either and am a real softie when it comes to my kids so bear that in mind!!
3.5 is quite young, the logistics of skiing are exhausting added with the communal childcare aspect, it being in a different language, even if they can speak English and there is a lot of crying and standing around at that age. However, if he is a robust child, used to full hours childcare and quite full of energy, he may well have a great time!!
My eldest skied at 3.5, went to morning ski school (cried nearly every morning as the lovely Family ski buddy took them off on the bus, stayed with them in the snow garden, picked them up when they fell over and gave them sweets if they cried) and had a couple of afternoons at the family ski nursery. He didn't enjoy the holiday and it put him off lessons, though he didn't like lessons, large groups, new people, new situations anyway!! He now skies amd snowboards well but resists lessons!!
My youngest found two hour lessons exhausting. We did Saturday afternoon lessons when he was nearly 4, he fell asleep when they stopped for a snack a few weeks running and sobbed for the rest of the lesson. At 4 and a half, he is fine!!!
If money is no object, perhaps a private nanny might be an option. It is more expensive but perhaps she might be able to take your DS to a couple of private lessons and then take him on the children's / beginners areas. If you manage to find a company who has other families in the area that week, they might be able to get together and have some fun otherwise it might get a bit dull being on his own all week. I have used Jack Frost Nannies in the past and they have always been brilliant, turn up with a box of toys / books / games for the week and all the nannies have been lovely.