Cle needs a 4 who can hit the outside shot, Amare a finisher around the rim.
Don't underestimate Amare's outside game. He doesn't really have three point range, but he can definitely keep teams honest from beyond the paint.
Not as well as Jamison can.
If we're talking about just this year I just don't see why Amare makes them better than Antawn would.
According to 82games.com -- 54% of Amare's shots are jump shots and he has an eFG% of 47.2% on them. Jamison takes 70% of his shots, jumpers, and only makes eFG% 43.9% despite a big bump from his three point shooting. Jamison also has a far higher proportion of shots assisted on (87% to 65%).
According to Hoopdata.com
(1) 10-15 feet -- Amare makes 53% of his jump shots while Jamison makes 37%. They take a comparable number of attempts per game from this range. Jamison has been consistently in that 35-38% range for three years while Amare varies widely year-to-year (38%, 51%, 36%, 53%).
(2) 16-23 feet -- Amare makes 45% of his shots from this range and has consistently hit 44-48% over the past four years. Antawn Jamison makes 40% of his shots and hit 33-36% in his previous three years. Again, they both take a comparable number of attempts from this range.
(3) 23+ feet -- Amare doesn't take any shots from this range. Jamison takes 4 attempts a night and has an eFG% of 55% on them. He has consistently taken 3.9-to-5.4 attempts and made 51-55% of them over the past four years.
In summary, Amare Stoudemire is the more effective midrange shooter + long two point jump shooter but doesn't shoot threes. Jamison, on the other hand, has a three point shot and is very effective with it.