Archlinux vs Ubuntu Core?

There are many differences between Ubuntu and Arch Linux.

With Ubuntu core, you get a ready-made distribution (based on Debian) aimed towards embedded devices.

Arch Linux on the other hand "is what you make it". After installing Arch Linux you are left with a minimal GNU/Linux system (not based on any other distribution). It is then up to you to configure the system as you want it.

To summarize; Ubuntu core is indeed tailored towards embedded systems, whereas with Arch Linux you will have to do the tailoring yourself.

Arch Linux link: https://wiki.archlinux.org/index.php/Arch_Linux
Ubuntu Core link: http://www.ubuntu.com/internet-of-things

Since this is an embedded project. I suggest you use either buildroot or yocto. Having used both, I further suggest you use buildroot. Its easier to get started with. However, if you have the time to come up to speed, pick yocto. Either is fine and although I am big fan or Arch and Debian, either of these two build systems are really the way to go.

I would say that both could be a good fit. But if you are going to do embedded development the answer might be tied to the platform you choose.

Example on a Raspberry Pi 2 you could use both. But Archlinux would be lighter weight in terms of disk space used, that's the disadvantage of snap packets on Ubuntu core.

Then you should try to see which software you would need and how they are readily available on both platforms. Finally given the power of many embedded systems, you could consider how viable it is to develop and possibly test from your desktop/laptop and then deploy on the target platform. How easy it is to do cross compilation from your main OS, etc. Snap packages might be "fattier" but are easier to deploy: they contain everything they require.

In the end there is no wrong choice. You need to assess your needs and then look at the best fit.