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.