Any recommendations for a beginner re using an impact vest versus a harness only? What are pros and cons.

Vests give newbies some extra float while they struggle to relaunch, they're nice for when you're trying new things too.
I'd skip on the impact harness dealio personally though, its nice to be able to run with or without the vest, and there are lots of options for harnesses that might be more comfortable for your body type.

I always wear an impact vest and always will. If you get hucked then you will at least not be as trashed as if you didn't have an impact vest. If you get one that fits well you shouldn't even notice you are wearing it. I know it has saved my ribs in the past. If you just want to ride back and forth and not do any tricks then an impact vest may not be necessary. It does also give an extra layer of warmth too. Just my 2 cents.

Cheers.