Engineering is all about choices and trade-offs.
If you want speed? Mirror
If you favor cost? Parity
Neither is better than the other. Ask the SysAdmin that is over budget but has too few users to appreciate the aggregate speed achieved by mirroring, or the SysAdmin that impressed his/her boss by saving money but now the company is going out of business because users are leaving because their site is unusable slow.
Also consider the performance during rebuild time. Both have some performance loss due to the fact that both require reading and writing one disk worth if information. On the other hand the performance loss is worse for mirroring (I think, I may have reversed that).