chisamba...you are a long time horsewoman. You will do just fine with whatever you take. If the clinician is so poor that he/she has no worthwhile insights for working with either green horse, then shame on them for taking your money.
A good clinician will challenge the rider plenty enough on the easiest horse. Nobody needs a challenging horse just to add to it and that is to say, unless the challenge is what the rider seeks to improve, never take a green and challenging horse to a dressage clinic. It is rarely worth the big buck you are paying otherwise. Oh and I have taken youngster to dressage clinics with a clinician I have cliniced for years on my "easy" horse. It was very informative and I learnt a lot but I have ridden with this clinician for years and we basicaly returned to the very basic of training scale, rhythm and relaxation throughout the session. I also wouldn't dream of doing three days - the youngsters will get a rush of adrenaline and then it drains out of them; on the third day they are done mentally and emotionally and the best thing for them is to just chill and not clinic.