This is very much a personal choice. Purebreds can be expensive but their future appearance and personality is easier to predict.
If you have set your heart on a pedigree kitten, take the time to do your homework on different breeds. There are over 70 to choose from. The Internet is a very good place to start your research, as most breeds will have a website devoted to them.
Find out about any inherited conditions your chosen breed may exhibit and what health checks they might require.
It's also important to ask yourself what sort of regular care your new kitten will need. Long-haired breeds such as Persian will need daily grooming. Do you have the time and the patience?
Mixed-breed kittens can be hardier, though it's less easy to predict what sort of cats they will grow into. If you can, visit your prospective kitten a few times and try to get a good idea of her temperament. Don't rush your decision.