You mentioned that you planned to develop the website. The key to its value is nearly entirely in the development, as neither domain has much, if any, real inherent value.
If you're asking which is better to go with, StressGuide.info would be my choice. However if you went the other way, ReduceStressNow also has the .net and .org available.
If you're mainly providing "information" about stress, then StressGuide would be a more suitable choice (and the .info would be fitting). On the other hand, if you're focusing on stress reduction tips or something like that, you could go the other way. Honestly though, ReduceStressNow really does not sound great as a name, in my opinion.