How long is your trip? If you’re choosing between one and two relatively nearby Central European destinations, this sounds like a perfect opportunity to wing it—go to Prague, see how much you like it, go to Vienna or somewhere else for a few days if you get bored.
I had seen all I needed to see of Prague in about four days, personally. The old town is very pretty but it’s also crammed with tourists. I’ve only been to Vienna on business for two or three days at a time and had the impression that if you were into it you could enjoy a longer visit very easily. So visiting both on a one-week trip in whatever proportion you cared to allot would be low-effort and probably enjoyable.
If you’re talking a much longer visit I think either could get boring after a bit—if you had more than, say, two full weeks I might consider adding a third destination.