Depends. Water temp is more accurate since water reacts to temp changes much quicker. You're right in that the oil will take longer to reach 'normal', but it will also take longer to reflect and change up or down.

Oil is also designed to work effectively over a large range these days, so even though it's not fully warm, it's still protecting.

Oil is also not pumped round the head to cool it, whereas water is. The water should be hotter as that's what cools the combustion chamber.

Basically, water temp is the crucial one!
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