I have a Sony 51" widescreen TV, component cables hooking up everything... HD satellite receiver, DVD player and xbox, when its down there (its usually up here in the computer room on a std. tv plugged in w/ RCA's).
I had originally bought the Madcatz everything in one adapter (has rca, svideo, components, stereo & optical audio + ALL cables, even components & optical audio) but for some reason it wasn't allowing me to set the HD quality settings in the MS dash, so I went and got the MS HD adapter. I would say that compared to good high quality HD broadcasts (ie. 720p, 1080i... DVD is crappy quality), xbox games in 720p or better (ie. TC:SOLA in 720p, Enter the Matrix in 1080i) are the same picture quality as TV. I didn't return the Madcatz thing b/c its always good to have an extra A/V adapter.
I also agree that Monster cable is waaaaaaaaaay overpriced for what it is. Its a cable. The way they price it it might as well come with it's own supercooler to make the wires superconductive and faraday cage to route the wires through to eliminate any possible chance of outside interference.
Also, just another POV on the subject. Will you really notice the (highly likely minute) differences between the two? Probably not. Especially when you're playing a game. You're concentrating on playing the game, not going "Wow, I'm sure glad I got that Monster cable, b/c man... this sure is great picture quality." By the time you did that you'd have died in the game
Just basing myself on roughly 3-4 week old Future Shop prices in my head, I know the MS HD pack is $39.95 CDN and the Monster 300 cable (components + RCA stereo sound, I think) was around $80 CDN. No option for optical audio either. The MS pack comes with a set of component cables with RCA audio, not the thickest sturdiest cable, but it does the job. It also has an optical audio plug, and if you have an extra fibre optic cable lying around, you're all set.
If I was you, I'd just buy the cheapest thing of known quality, so the MS one. Save your money, don't waste it on cables.