The HDRetrovision cables convert the RGB signal to Component so yes they will look better than a composite signal. However like you discovered they don’t make them for every console. So depending on your needs they may or may not be a solution.
There are pros and cons to both the Framemeister and OSSC, and I would recommend one or the other based on your needs. There are compatibility issues with the OSSC + SNES and I have heard PC Engine with a lot of tvs. I personally think if the best you can get from the OSSC is 2x mode into your TV, its a waste of 200 bucks. Most tvs will produce a softer upscale (since they’re meant for live action video not sharp pixel 240p video games) than a proper gaming upscaler will. It also only supports SCART RGB, Component, and VGA. If you have any consoles that can’t output one of those signals you’re just out of luck here. I know most consoles can be modded now adays to provide one of those, but there are some that lack any kind of mod. The 3DO, Atari 7800, maybe the 5200 as well. Colecovision RGB mods are just coming along and not readily available. So there are consoles it won’t work with.
On the other side of the coin though, the OSSC has no lag other than what your tv produces, and has better color handling than the Framemeister. If you can get the higher line modes to work the OSSC will look absolutely amazing. The OSSC also handles resolution changes (ie going from 240p to 480i or vice versa) faster than the Framemeister does. This mostly affects the PS1/N64/Saturn generation, where a lot of games use 480i for menus and such.
The Framemeister will add around 16ms of lag on top of what ever your TV produces. Though it can take, composite, S-Video, Component, and RGB. It also allows for up to 20 profiles per memory card. FirebrandX has created amazing Framemeister profiles for MANY of the systems out there. If you need more than 20 profiles you can spread them across multiple memory cards. Which is what I did. Some systems use multiple profiles for games that are progressive, ones that are interlaced, wide screen etc etc. I run 15/16 systems into my Framemeister. Mostly this again is for the PS1/N64/Saturn gen. I also think the framemeister is a better Deinterlacer than the OSSC.
The OSSC does support profiles, and some one correct me if I’m wrong here, but I believe it only allows for 9, and you can’t name them directly.
As you can see like I said there are pros and cons to both items. You need to look at your needs and what you’ll be using it with and go from there.
I would first start by going to rtings.com and research tvs. They do extensive testing, including lag tests on various inputs and with different settings turned on or off. My method is to usually decide how much I can spend, what size set I want and what things do I prioritize. For me it was low input lag and color reproduction, so I spent time on rtings comparing various sets in my price range.
I currently run my framemeister into a Samsung UHD tv with HDR. I keep HDR turned off, and game mode on, on the input I run the framemeister into, since HDR does nothing for classic games, and it adds more lag.
I know this was a long post but I hope it helps. Basically you need to figure out your needs, pick a tv, and check the compatibility list, then decide from there which works best for you, be it the OSSC or Framemeister.