I work professionally as a director of photography and colorist so I'm more in VSCO's market and can explain their product better than a lot of android enthusiasts. I used to use Snapseed almost exclusively on my phone, I now split my time between the two apps.
Snapseed and VSCO are trying to accomplish two different things when it comes to image editing. VSCO is a film emulation app first and foremost (which has always been their big sell) and photo editor second. Snapseed is entirely a photo manipulation app. Let me explain what I mean.
VSCO is packed with free and pay filters (their business model) that are designed to emulate specific film stocks. You've seen those pictures shot on certain films (Kodakchrome or Technicolor are particularly well known examples) that have a certain "look" to them? VSCO has "looks" that are designed to emulate those exact stocks. The "x" series for example are designed to look like TriX Black and White films, the "p" series are polaroids, etc. The idea is that with a single filter, you can get 90% of the way to the look you want with the knowledge of what that film stock should look like in a similar situation.
There are a number of tools to edit once you've selected a look (or you can bypass a look and jump straight to editing). However, these tools are more limited than what you would expect on a typical editing suite. There are basic options for saturation (though you can't turn it from color to b+w), contrast, exposure, etc but they add a couple of look emulation specific tools. A "fade" filter gives that washed out, hipster look you've seen overused to death. Highlight and shadow save filters give you more control in those areas than either snapseed or pixlr. There is an even a setting to add film grain to really nail your emulation. With that said, it's not intended to be a full fledged editing app as there are some major tools left out.
Snapseed is 100% photo editing. They give you all your basic tools, as well as spot adjustments, fine control in your vignettes/blurs/spot corrections, etc. These are the things that VSCO lacks that make Snapseed a true editor. With that said, Snapseed has plenty of their own presets, though these are little more than glorified Instagram filters. The application is coarse and extremely broad stroked generally giving poor results (if the results are usable at all) and require lots of edits to make acceptable.
This is a big wall of text, and I can elaborate as I need to, but I thought it would be easier to just give you a visual representation. I snapped a photo of a car I saw on the side of the street nearby. I shot it on a Nexus 5 which is generally considered to have a fairly mediocre camera. Lighting was pretty decent. This is the original image. This is VSCO. This is Snapseed. This is an album of all three. I messed up the crop on one of them, apologies for that.
Let me know if you want to know anything more!
TL;DR - They do different things. Look at my example images.
Original:
VSCO:
Snapseed: