Processing is much faster than 15.5 months if you have a straightforward case. I applied under the 3 year rule and my oath ceremony was about 4 months from the N-400 application date. I think my case was straightforward (solid evidence of marriage & co-mingling of finances, no arrests or criminal history, etc.) but I may have just gotten lucky with the timing. Hopefully four months (or less) becomes the norm!
3 vs 5 years doesn't matter as long as you're eligible. If your wife wants to be a citizen, I don't think there's any reason to wait. We thought the online N-400 application was the easiest application in the process. There's a screening section to check if you're eligible to apply and under which criteria (the 3-year rule or 5-year), the application automatically skips any sections that don't apply, you don't need to worry about putting extra info in the addendum pages, you can save your work and come back to it, and the submission notice generates shortly after submitting the application.