Would you pick diamond or white sapphire for your engagement ring?

I have seen a lovely white sapphire ring for my engagement ring, but my partner is saying for the cost of the ring we could afford a decent diamond...and wouldnt i rather a diamond?
any thoughts on this?
Update: just for the record i love this sapphire ring, it is stunning and different! every girl i know has the same old boring solitare diamond ring!!
Update 2: the sapphire is natural and has paper work clarifying it has been sourced in sri lanka etc
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