I think it depends on what you're looking for. The Acer C720 is a better version of the Acer C710; it is probably the best budget model chromebook available right now. (We'll see what Asus releases in the near future.) Having said that, I'm unsure you'd really need to upgrade from the Acer C710 if you want to buy the Acer C720. By contrast, the Samsung Chromebook 2 will be a more expensive and likely better model; both the 11.6" and 13.3" versions will have 4GB of RAM (there is a version of the Acer C720 that has 4 GB but it is hard to find) and the larger Samsung Chromebook 2 will also have a nicer screen. On the flip side, both models will be more expensive than the Acer C720 and also will have new, previously unused ARM processors; the latter difference is a real wild-card.
Considering you already have the Acer C710 and the Acer C720 wouldn't be that different, in your case I'd recommend buying the Samsung Chromebook 2.