Hey MM definitely IMO go with LN2. I find it so much easier to wrk with . As for holding a temp that just takes some practice.
Dewar size I have a 35L which is good for a day maybe 2 depending on what I bench but it weighs about 70lbs full so I wouldn't go much bigger. You may want to check local supplier prices before you go too far with it. I get mine filled for about $40 but some places aren't as friendly.
EDIT: just had a look a some of your posts and it seems you were having some real issues. Did you get the extension for the pot? Cause like Witchdoctor said it's a heavy poy, I have one of the older ones but very similar. You should have been able to take it down to -70c without issue and keep it there, I also don't understand what you meant by the "board freezing"