Considering the 550 is more powerful and overall a better machine, I'd advise to go there, though you need to make sure you have a warranty for a used machine, or that you trust the seller. Chromebook are resilient but a broken machine is a bad deal.
I have the Samsung Chromebook and am very happy with it. Screen size, keyboard and overall weight are all good compromises. It boots within seconds and runs very well. As I am a very impatient person this is important to me. Battery life has been as advertised, 6-6 1/2 hours. Recharges very quickly. Have been streaming Downton Abby via Prime on it full screen HD and is fine. It is very light weight. No moving parts, no fans, no noise, snappy and nice keyboard.
I've had the Samsung Chromebook for over a month now. I use it daily, all day, without a battery recharge. It's light enough to carry in my portfolio. The reason I purchased it was for the weight and the battery life. I would highly recommend it as an everyday on-the-go notebook. No noise. Quick startup. Great to use in meetings.
Well if they are both similar in price go new but I got a used one for 200 and it works great. Im typing on it now haha. If you just wanna get on facebook or even use this as a college notebook this is a FANTASTIC computer. No apple crap but still user friendly.