Ideally you'd want both but (in my opinion)
Oil temp gives a much better indicator of when you're engine is at proper operating temperature (unlike the coolant temp gauge) as the oil takes a lot longer to warm up and conversely when you've been running the engine hard you'll see the oil temperature going up, you don't want to exceed ~115/120DegC mark for too long because the oil can break down and be less effective. It also gives you a built in "turbo timer" because you'll know after a hard run when the oil has cooled back down to "ordinary" temps before turning off the engine. It gives more helpful measures for day to day running of a car.
Oil Pressure also gives a good indicator to when the oil has reached operating temp because the pressure will drop as the oil warms up. But then really it just sits at the same reading day by day until you have a pretty major ****ing
problem and the pressure drops because you've either dropped the oil out the engine or you've cooked the oil (which you could prevent if you had an eye on oil temps). The pressure
will increase if you get debris blocking a journal for instance (!) Very handy... but then it will be a marginal consistent pressure increase, which you might not notice anyway until it causes a major
problem.
They kinda do the same but different... I'd go with Oil Temp personally.
