the primary difference between the OSSC and the Framemeister, ignoring cost and aesthetic, is that the OSSC is a linedoubler and the framemeister is a scaler. What that means, for the end user, is that the OSSC takes an analogue signal (say, 320x240p via euro SCART), then multiplies each axis some number of times (2x for 480p, 3x for 720p, 4x for 960p, 5x for 1200p) and converts it to a digital (in this case DVI) output. The OSSC offers a pretty large array of settings for optimizing this process, but everything it does essentially boils down to this. Well, it also offers bob-deinterlacing and pass through options, but those aren't why you're buying it.
The framemeister, on the other hand, can scale just about any input to various DVI and HDMI standard resolutions. Instead of simply multiplying the axis, the framemeister allows the user to stretch and manipulate the image however they like. This is why the framemeister's options menus are a rabbit hole and there's such a community for optimal settings profiles.
In the end, the value of both is contingent upon what's important. Barring the unfortunate existence of some noise in the Framemeister's inputs and processing, it generally can produce a sharper (or less sharp, if that's what you want) picture. However, you're looking at a pretty consistent 1.5 frames (~20-30ms) lag across all content. While this probably won't bug you if using a low lag display, on a monitor that already has a few frames worth of lag, it'll really start to add up. Also, the framemeister offers impeccable compatibility in terms of output, since it essentially normalizes the input signal so it conforms to HDMI standards. This is something that the OSSC doesn't offer, and the inconsistent compatibility is the result. To balance this out, many people (not including myself, i'm rather ambivalent) think of the OSSC as being more plug-and-play, and therefore an easier solution for getting their retro gaming fix with quality on a modern display.
Really it's a matter of what you want. Read Fudoh's site. Read the Shmups thread. Read the VGP forums.