Both are upcoming spinners, and both will going to lead the spin attack of their nations soon.
Swann has played 12 matches with 48 wickets at 30.4, and had played his matches on helpful tracks, but against good players of spin (other than WIndies) bar first few Ashes matches. He possesses a vicious off break and a good arm ball.
Herath had a patchy career for understandable reasons, and had taken 59 wickets @ 34.4. Most of his matches were also against quality players of spin (ex. Australia, Pakistan). Can spin it on helpful tracks, but has lot of subtle varieties in the bowling.
Swann is the better batsman, both aver very good in the field, but Herath may be touch better with catching.
Who do you want as you spinner?