The only GB for sale until April 21st is the Verizon version with LTE. Yes, after the 21st you should be able to get the 8/256g Wifi only version.
I get all day with plenty of battery left away from the desk. I haven't and don't intend to use it as a desktop so I don't really know how many hours I can sit in front of it with MS office programs running.
I think it really comes down to whether or not you want a tablet or a portable desktop computer. Not to take away from the Samsung but it can't compete with the Surface pro 4's top end specs and IMO the SP4 cant compete with the tablet feel of the GB.
I'll admit I was having a bit of buyer's remorse (even though I have till the 28th to return it) based solely on the fact that I only have 4g of ram but the way I use this it is more than enough.
If you watch Netflix and Amazon
a lot, the GB's HDR display is a sight to see, it is amazingly and surprisingly better than non-HDR. Speakers are very good for movies IMO as well.
Depending on where you live you might be able to find one at your local Verizon store where you could play with one...then if you like it and don't want the LTE you could get the 8/256 one at BB or Amazon
when they come out.
But if you like to load up your computer with tons of running processes then I think a loaded SP4 is a better fit.