The U-test is not a test for means.

You can use the Welch test, which is a t-test corrected

for unequal variances. Since both samples are about

the same size (I suppose), Welch and t-test won't

differ much.

Whether the data (within groups!) is normally distributed

or not, doesn't matter for the t-test with such a big sample

size such as yours.

With kind regards

K.