I like the 6" scale better because they are a lot more detailed and have a sturdy construction.
Unfortunately I missed out on the whole Marvel Legends because I was collecting other series at that time and still had most of the 4.5" Toy Biz X-Men, X-Force, Spider-Man & Iron Man figures.
I ended up selling all those old figures a couple of months ago because I started to look very critical at most of my collections.
Regarding MU figures, I have to agree with Aesthetikz on many points, that the MU are flimsy, wimpy and weak~fragile figures with the lack of detail capabilities, They cost too much for so little, lack of pack-ins and accessories, no BAF, Etc...
What I am mostly disappointed in, is the fact that Hasbro made the MU T-crotches out of very fragile PLASTIC,
while the SAME figure construction as Hasbro's G.I.Joe figures have METAL T-crotches.
Hasbro knows how to make them better, yet they make them very fragile! I find this inexcusable from Hasbro's part.
Aside from that, I actually started to collect the Marvel Universe instead of Marvel Legends because of various reasons;

- MU are easier to find
- MU are "cheaper" (but still expensive at almost $10 for a tiny figure) since ML are mostly on the secondary market only.
- MU are in scale with many other action figures, which is the best possible reason why I got MU instead of ML because it's in the same scale, it has that awesome cool factor.
The figures I have in the 3 3/4" (4") scale:
- Marvel Legends Showdown aka Marvel Superhero Showdown by Toy Biz
- Marvel Universe (Fury Files, Hammer Files, Secret Wars 25th Anniversary) by Hasbro
- X Men Origins Wolverine by Hasbro
- Iron Man 2 (Comic Series & Movie Series) by Hasbro
- DC Universe Infinite Heroes by Mattel
- G.I. Joe, the 25th Anniversary by Hasbro
- Star Wars (modern) by Hasbro
- Star Trek, Galaxy Collection from the new 2009 Movie by Playmates
- Indiana Jones by Hasbro
- Pirates of the Caribbean by Zizzle (pirates and sea creatures!)
- Terminator Salvation by Playmates (because it's always cool to have some endo-skeletons!)
- Chronicles of Narnia
- Microman (various characters from various licenses) by Takara
- DragonBall Z Hybrid Action Figure by Bandai
- Hellboy 2: The Golden Army by Mezco
- Cinema of Fear by Mezco
- WWF Deluxe Build N' Brawl by Jakks Pacific
- Various Japanese Gashapons/Capsule Toys
I am not exclusively collecting the 4" scale figures, on the contrary, I'm collecting many other series ever since the early 80's!
My main series atm is Masters of the Universe Classics.
I still want the main Team of X-Men in the 6" scale Marvel Legends. I started last month with Nightcrawler, Iceman, Psylocke and Longshot.