Doubt: Mann Whitney or Wilcoxon test?

Hi. I did one sample of 131 questionaires out of a group, and am then making 2 sub groups out of those 131 responses based on a demographic variable (years of experience). Is the comparisson of them a comparisson of independent samples or between related samples? I beleive they are independent, since assocaite the dependent case with pre and post tests of a same group, which is not the case. I need clarification as the statistic test for independent would be Mann Whitney whether for related would be Wilcoxon