Maybe look at it from a time away point of view, as you're looking at three very different time commitments:
G2- almost 2 months away from home
Logan, St Elias, Ama Dablam- 3-4 weeks away
Vinson- 2.5 weeks unless flights get weather delayed
I'd agree with Damien- if you had a good experience on the West Buttress, Vinson will probably feel similar. The work days are close in duration, with the "new" route to high camp being a pretty good day, similar to moving to 17.2 on the Buttress. One big difference is that down in Antarctica, you really need to take charge of your personal well being, whether you're with a guide or not down there. It can be (but not always is) significantly colder overall than on Denali, and you really need to be deliberate with your self-maintenance.
I'm not sure why someone would suggest an alpine ice course before letting you join them on Vinson, unless they just wanted to get another trip out of you. I also don't see it so much as a "lateral move." Antarctica is one of the most amazing places I've ever had the opportunity to visit. It has a tangible feeling of being "out there- waaaay out there!" That difference alone would stop it from being a lateral move. in my opinion.
I'll not take up the guided/non-guided debate, as I'm biased. Good on you for doing your research and trying to get some adventure in before the kiddos come (my best adventure to date, personally). Have fun with the process!