In my opinion giving us the match at Wrestlemania would make it extremely clear that the company doesn't hold long term booking in the same regard as they once did. I think that seeing that match at Summerslam would be better or even at Survivor Series as it would help the match have more meaning and background. Also holding the match off until August or November would allow them to have a major match on a PPV not called Wrestlemania.
The one thing I don't understand is that the WWE has really tried to make both Survivor Series and in particular Summerslam feel like major events again in recent years. So with that being said they should try and long term book for at least one of them giving us major matches to make it feel like another Wrestlemania. Fact is I think it would a hugely missed opportunity if Ambrose and Rollins collide at Mania instead of later on in 2018.