Stick with 18V Li-Ion or go to 20V?

Hey everyone… first post here.  I searched around and didn’t immediately find a post like this, but I’m sure it’s out there…

I have a decent collection of Dewalt cordless tools that utilize the now old 18V Li-Ion and even older 18V Ni-Cad batteries.  Knowing that the 18V Li-Ion worked with all these tools, I was very happy.  I just need to bolster my 18V Li-Ion battery inventory and I’d be set.  Seemed like a moment later everything was moving towards the 20V Li-Ion. 

So here are my questions…

1. I can only assume that without modification, the new 20V Li-Ion will not work in my 18V tools, right?

2. Is the 20V THAT much better than the 18V that I should completely upgrade my tools? I just find it hard to believe that 2V of Li-Ion is so much better.

I would much prefer to just adding more 18V Li-Ion batteries until my tools start to crap-out.  THEN maybe consider going to the next best thing…

Thoughts?

I got a dozen or more 18v bare tools right now.  No way am I going to sell them & switch to 20v.  I do have some 12v Max stuff for lighter tasks, works well.  The batteries on 20v Max slide on, where 18v push into handle.  I have tested my DC9180 18v LI-ION batteries right off charger on a digital multimeter, they are at 20v.  Maybe the 20v Max would be the thing to pick if you are starting off without any cordless tools.

I am too invested in the 18V system to switch. The only other tool I would consider getting is an oscillating multi tool if/when it comes out. If I need batteries I’ll go to Home Depot and get the 2 pack for $99.