The Lensbaby Spark is the lowest priced way to get into a Lensbaby piece of equipment. Right now the MSRP is $80 USD. The LensBaby Muse on the other hand is around $150USD. You would think for those savings you would be missing something, and you are right. The Muse comes with the Lensbaby Double Glass Optic, the Spark does not. Believe it or not, the Lensbaby Double Glass Optic costs $80USD on it's own, so it is pretty much a wash if you are going to buy the Spark to later upgrade and buy the Double Glass Optic.
What this really means is that with the Spark you are limited to a single aperture of f/5.6. On the Muse for example they note aperture values of f/2, f/2.8, f/4, f/5.6, f/8. Something doesn't quite add up for me, as the Double Glass Optic that comes with the Muse notes fewer apertures then the stand alone Double Glass Optic, so potentially they include fewer of the magnetic discs(control aperture) with the Muse then if you buy the optic alone.
What else is different besides the optic and aperture control? The Muse is has many more mount options, so if you aren't Canon or Nikon mount, the Muse might be your only option. Ascetically they are a bit different, the Spark has a huge ring with see through lettering - not my favorite. The Lensbaby lenses aren't really meant to look professional either though, you are going to be fidgeting with them and look kind of ridiculous no matter what - unless you have the Sweet optic 35 I guess.
Anther potential difference is how sharp these get. The Spark will do a good job of selectively focusing an area, or blurring out the outer edges of the image - but I do not believe it will get anything in very sharp focus. The Double Glass Optic can get things in good sharp focus, but I am guessing the Spark is more like the Single Glass Optic which really isn't sharp or great contrast on anything.
If you want to know bang for your buck, I'd get the Spark. It's half of the price and will do selective focus well. It is fun and might bring out your creativity. If you want sharp optics in a similar range that has a variable aperture, I would get a 50mm f/1.8 lens - that is a great bang for your buck in a similar focal length but with great sharp optics when considering the price.