Too late for Prague. I've been there three times, 1990,1995 and 2005.
The first two times it was great and unspoiled. By 2005, Wenceslas Square had Debenhams,Marks and Spencer, Tesco etc, several nice restaurants had gone to be replaced with Mcdonalds and Kentucy fried rubbish. Sex show touts were a pain in the ass, and the place was full of English Hen and Stag party drunken idiots. The Prague town council should have controlled things better, they will kill off the golden goose at this rate.
Budapest is still nice, but would suggest Paris and Rome.