Hi Mitch
When you've gathered opinions from here, as you're looking at a significant capital purchase, ask for a demo of both. You'll need to consider things like noise, serviceability and access to components, perhaps the weight of the COMPLETE system installed on your van etc. etc.
If you're looking at previously owned, if you're quick enough, Dave Roelents has a 421 for sale and it is immaculate, converted to twin wanding, and has, I believe, an LPG option should you want it.
Good luck
Safe and happy cleaning

Ken