I recently bought a ultrabook and needed to decide between the Acer and the Samsung one. So I'll give you the run down between the two.
From what I've read the i7 upgrade is usually not really worth it, what you can in speed you lose in power consumption and heat generation. So I consciously went with the i5. However there are a few other things to note about these ultrabooks and you're prolly best of if you can get a sample one to try at home for a few days and then return it at no charge if you dislike it.
I first bought the Acer Aspire S7 as it has more RAM and a RAID SSD system (which I would have decoupled to get double the capacity). However I found that the keyboard made a constant weird noise if the backlight was on, which annoyed me. This, Acer told me, is by design and not a fault in production.. In addition the keyboard was quite unresponsive, in particular the space key quite often simply didn't work. There were a few other things that annoyed me like battery life, but in the end the keyboard really was the deal breaker and I returned it after 2 days.
Instead I bought the Ativ Book 9. Now, the keyboard is much much better as is the battery life. However I find the noise of fans on the laptop quite annoying and it seems the vents are unfortunately placed... Use it in your lap or on a couch and it'll run hot almost instantly. However when the fans aren't running (eg if you're using it on a desk as you're supposed to. It's super quiet). So I kept this one (also because it is the cheaper option).
In addition the screen is quite loose and wobbly and in my case is getting more loose and wobbly over time.. I fully expect to have to send it in to have this fixed before the end of the warranty now.
It goes without saying that both are super fast and pretty and awesome. There's no clear winner spec or price wise and I would even go as far as to say it's down to personal preferences... So try them out, in a quiet environment (if you can). I heard neither the sound of the keyboard backlight nor the fans in the shop... but I found it really annoying once I was sitting at home. Yes, that is quite picky, but for the price I think you can be.
I actually have this one: http://www.microsoftstore.com/store/msusa/en_US/pdp/Samsung-ATIV-Book-9-Plus-NP940X3G-K03US-Touchscreen-Ultrabook/productID.283976900
(paid full price for it too
I haven't run into the RAM limitation yet, so depending on what you do with the machine, you could save $400 by choosing the one with only 4GB of RAM.
This being said the RAM is usually soldiered into the MB with these machines, so you can't upgrade at a later point.