Bit confused here, as the Spectre I'm seeing around $400 while the M6 seems around $700. Are you looking at a used M6 - those seem around $400? I'd choose new over used every time due to the inherent risk but obviously a used machine at $400 is more powerful than a new machine at $400 assuming fair pricing on the used machine.
If I'm correct, then the M6 is an i5-6200U part (2.3GHz 2C/4T) while the Spectre uses an M3-6Y30 processor (up to 2.2GHz 2C/4T, 900MHz base). These are two wildly different parts at two wildly different targets which is why they're so hard to compare. For performance, obviously the i5-6200U wins. However it can use up to 24W TDP while the M3 is capped at 7.5W. It's as wide a power gap as a mobile CPU vs a desktop CPU. The i5-6200U should lead to a hotter laptop with shorter battery life and more weight. The M3 should be comparatively light, cool, and have good battery life. It should be more portable and better suited to being away from an outlet for long periods of time -- the original reason laptops were invented to start with.
Now, a 128GB SSD vs a 1TB 5400 RPM drive is a preference issue. The SSD would be faster, leading to a boost in performance especially coupled with a weaker CPU that needs as much help as it can get. However you're limited to 128GB, and you have to figure Windows is going to want ~25GB of that. So it's like comparing 100GB usable vs 975GB usable. If you only intend to store typical office documents and mostly stream things then 100GB is probably fine.
And the RAM. Obviously 8GB is better, and quite honestly 4GB isn't useful unless you can upgrade. Find out if you can upgrade the Spectre to 8GB or not. If not, I'd say that it's not something you should bother with and that leaves you with just the M6 as an option unless you compromise on some other aspect which led you to these two laptops to start with. If you can upgrade the Spectre to 8GB then you need to weigh your need for performance versus the physical aspects of the two machines. With the M6 you would be better off with an SSD inside, but typically this is something you can swap out as a 1TB HDD means a standard 2.5" SATA disk so it's simply a question of how comfortable you are with ripping open a new laptop.
At a glance as well, the M6 has a bigger screen than the Spectre but both are 1080p. Again this goes back to portability, and weight. If you intend to use it as a proper laptop then the smaller lighter one is inherently better, to a point. That point being that the screen isn't so small you can't read anything. Assuming normal eyesight, the Spectre should be a bit small but fine to work with. It may get annoying or taxing after long periods of time. If you intend to have this as a mostly stationary machine, eg used on a desk in a college dorm and brought to class on rare occasions then the size, weight, and heat of the M6 may not matter much to you and the bigger screen + keyboard would be easier to work with.