As you already have a decent SB, I would go with 950 as both are same panel. If you later upgrade to an Atmos SB it allows you to choose the one you want. Result should be better than a blade speaker but I have not heard the GZ1500, so just my opinion.
I use the 5.1 surround system with my 65 inch GZ950 it got turned off by mistake the other day and my wife noticed the difference straight away she said it sounds a bit tinny its probably ok but I'm used to proper surround system I had a Yamaha soundbar before never used the TV speakers for years