I have used both Alpine Ascents and RMI. I had a good experience and would reccommend either one.
I did the 8 day Denali prep course with AAI. It was first class learning experience. We were on Mt. Baker, WA. The first few days we hiked for half the day and then had school in the afternoon. Once we reached about 9,000 we set up camp and stayed 3 nights. We did the whole winter camp thing. Built snow walls , etc. They also did an extensive amount of self arrest training with the ice axe. We had a great steep snow slope that flattened out with no obstructions. The wildest ride was be dropped over head first on your back. You really build up speed before you got yourself turned around and stopped. It was also a good workout climbing back up each time. They also spent alot of time on crevase rescue. How to set up the pully system and the proper way to get someone without further injury to them. Some of the afternoon session were on mountain phisiology, compass & map reading and using the avalance beacons to find someone. Then we moved our camp to about 10,500 and tried to make a summit attempt. We awoke to winds and snow. We set out that morning and were eventually turned around. It was almost better than making it however as we had to use many of the skills we learned about travelling on a rope team and setting the flags to find our camp on the way down. Only caveat is to be in the best shape of your life as it was a cruling 8 days. My only wish is that I started this when I was alot younger. I would rate them a 10/10 for this.
I used RMI for Rainier. They do a nice job for what it is. There goal is to get you up and down the mountain and its done in 2 days. You have to do a mandatory 1/2 day training course, which hits the highlights of traveling on a rope team, walking in crampons and self arrest. I must say that after the self arrest training I had with AAI the training with RMI doesn't compare on this item. They were very serious about the safety of the trip. They were constantly evaluating the performance and shape of each climber and were not bashful about telling someone that they were becoming a liability and should think about going down. They have a bunch of guides going up with the group so they can take people down if needed. I did the Disappointment Cleaver route and we started with 19 people and 7 summitted (me included
). Most turned around at the top of the cleaver which is around 12,000. Part of the problem with this trip is there is no time to aclimatize. You spend the night at Camp Muir which is 10,000 but your up at midnight and on the trail by 1:30 or so. The second day is also the longest. Climb the last 4,500 feet and 4mi or so to the summit and then all the way down (9,000 ft and about 9 miles). you get back to Paradise Lodge around 4 PM. Both RMI and AAI do a three day climg up the Emmons Glacier. AAI has a very limited number of permits and the trip fills up in about 2 days when they open it up. RMI has the majority of the permits for the mountain. There was a move out there to get that changed to allow the other Guides more. I don't know if it has changed or not.
The effects of altitude seemed to effect everyone a little different. Two of the people on my rope team that turned around were in there mid twenties and in great shape (marathoners). They made it to about 13,000. I did Kilimanjaro 19,340 (10 day trip) last February and found the first night at 12,000 was where I felt the most effects. after that it wasn't to bad. My wife , who doesn't do much climbing at all, made it without to much difficulty. She used Diamox and i didn't.
Hope this helps.
Chuck