As someone who takes advanced group lessons about 1-2 times per year (I often end up as the only student at my level), considering you're going to Keystone, a major CO resort, I would probably recommend the group lesson. I think it'll be the best bang for your buck.
My experience is that at most major resorts with reputable ski schools, like others have stated, the instructors teaching the advanced lessons are usually more experienced and more highly certified. If you do take a group lesson, arrive 5-10 minutes early for your lesson and find the ski school supervisor. Tell him or her honestly they type of skiers you and your wife are (skill level, terrain you spend most of your time on, level of fitness, agressiveness, etc) and what you'd like to get out of your lesson. Be as specific as possible. Also, request a PSIA Level 3 certified instructor.
I haven't been to Keystone in many years but from it's reputation, it is not a resort that is known for it's advanced/expert/extreme terrain. It is known as a great family resort. This will likely work to your advantage as there will probably be less advanced skiers taking lessons at Keystone. However, you mentioned that you are self taught. When you and your wife do the ski off, your self taught technique (unless you two are naturals) will be easily identified by the ski school supervisor doing the class assignments. This might end up with you being placed in a lesson at a lower level than you might think. It all depends on how you ski and how you're evaluated. The holiday weekend also works to your disadvantage.
Yes, as JASP has stated, by taking a group lesson you won't be able to pick your instructor. What I've done with some success in the past to address this issue is to find out the names of about half a dozen good instructors at the resort you're going to (post a question on Epicski or ask JASP). When you talk to the ski school supervisor before your lesson ask whether one of those instructors is teaching group lessons and if so, request to have him/her as your instructor. This has worked for me probably 20-30% of the time I've done it.
If you aren't satisfied with your lesson inform the ski school either during or after your lesson. Talk to the supervisor or manager. Again, be specific about what you're not satisfied with. At a place like Keystone, if you're unhappy for vaild reasons they should be able to make it right.
If you are satisfied with your lesson, don't be shy in letting your instructor know. Also, depending on how satisfied you are and how many other students were in your group, don't be afraid show your appreciation by tipping your instructor. There has been much discussion on Epicski about tipping your instructor. My general rule of thumb is for an average quality group lesson with 3-6 students including me I start with ~10% of the cost of the lesson. If it's a 2 student lesson I usually start with ~15% and for a 1 on 1 group lesson I usually start with ~25%. I have tipped up to 50% for an exceptional lesson.
Hope this has been helpful. Have a great trip!