Without knowing what Sempron or Athlon it is hard to say. But out of the same generations the Athlon is the better chip than the Sempron. Then again a new generation Sempron is better than many older generation Athlons!
Just to clarify Athlons have been around a long time and used to be as low as around 600MHz single core and are still sold new in 2, 3 & 4 core versions while the old single core ones are history. Semperons that are still sold are single core versions of new athlons so are like between half and a quarter of an Athlon. Though a new Semperon chip is better than an old single core Athlon.