Eh. Tough question.
I'm not exactly a professional at either Melee or Brawl, nor do I play competitively, so don't expect me to know about either extremely well.
I'd say I prefer Melee Marth. He seems a bit tougher than he is in Brawl, and, among other things which I cannot currently think of, his side-b can be used as a recovery similar to Mario's cape, whereas in Brawl he simply stops.But again, I likely don't have a clue as to what I'm talking about >_>
That seems pretty accurate. Brawl Marth can recover with a charged B though, and it works quite well if you get the time to charge it.
Melee Marth has longer range, and his tipper hitbox is bigger too (or it's more powerful, I can't tell for sure), so I would say he's tops.
I don't know about that second question. I never pay much attention to tiers or most competitive SSB stuff, but if I had to guess, I'd say Melee Marth is probably lower- to mid-high while his Brawl version seems more like upper-mid.
But again, I likely don't have a clue as to what I'm talking about >_>
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Well, right now that's pretty much spot on. Marth is tied with Falco for 2nd in the most updated Melee tiers IIRC, and he's near the top in the Brawl tiers. Tiers are misleading though, because they are based pretty heavily on usage, as it directly affects that character's metagame. More usage = more developed game = higher tier ranking in general.