The Slater who played the first half of his career (1993 until he was dropped), I would take over Langer anyday, he had pretty much the fastest, most correct footwork in world cricket, a technique without any notable flaws and a thrilling range of shots, he was a real flair player and scored with great consistency for one who seemingly took so many risks, averaging at or close to 50 for his first 3 years in test cricket.
In that period he played 34 tests and scored 7 hundreds, averaging 47.4
However during the second half of his career Slater was not the same player, there were still flashes of brilliance, his hundreds against New Zealand in 99/00 and England in 98/99, tearing England apart in his first innings of the Ashes in 01 and a solid series against the West Indies in 2000. However he was not nearly as consistent or convincing as he had been earlier in his career, seemed far more emotional and prone to rash shots under pressure. Flaws began to crop up in his technique and footwork and this was all compounded by the well publicised off field drama he was going through.
At their respective peaks I believe Slater was a better batsman than Langer, but taking their entire careers into account Langer has achieved more as an opening batsman than Slater did, I'm not voting for either.
In that period he played 34 tests and scored 7 hundreds, averaging 47.4
However during the second half of his career Slater was not the same player, there were still flashes of brilliance, his hundreds against New Zealand in 99/00 and England in 98/99, tearing England apart in his first innings of the Ashes in 01 and a solid series against the West Indies in 2000. However he was not nearly as consistent or convincing as he had been earlier in his career, seemed far more emotional and prone to rash shots under pressure. Flaws began to crop up in his technique and footwork and this was all compounded by the well publicised off field drama he was going through.
At their respective peaks I believe Slater was a better batsman than Langer, but taking their entire careers into account Langer has achieved more as an opening batsman than Slater did, I'm not voting for either.