Antidepressants sertraline and citalopram?

I was taking sertraline for approx 1 year then stopped due to nausea, i have just started it again as it did help marginally but my GP has suggested citalopram. Just wondering what peoples experiences of citalopram is, especially those who had first tried sertraline. I've also tried fluoxetine to no effect. Also has anyone found escitalopram to be more effective than citalopram?