I just wanted to add that when I tried the 8GB model for a couple weeks, which was when it first came out - so no new system updates since then were in use, which could speed things up a bit I suppose. But again, overall even just with a single browser open such as Safari, if you open a bunch of tabs - the machine will slow to a crawl.
I dealt with that type of simple and significant slowdown for a couple weeks. I knew it was a whole new system going into it and that it was somewhat of a "test" unit so that wasn't a huge shock, though the 16GB version I replaced it with is superb and can handle exponentially more tasks and makes using it a joy and not a slow walk which is what you'll get with the 8GB.
Go with your gut. And if you do get the 8GB model, definitely run it through everything you use and throw it all at it daily for a couple weeks. If it doesn't perform well, return it and get the 16GB model. At least for myself, that single swap was the best move I could've made and love it. I'm just sharing my experience here, as well as exactly the same experiences that a few friends went through, all of whom ended up doing the same thing I did and swapped it for the 16GB and never looked back.