If you could only get one or the other (oil temperature or oil pressure gauge), which one would you choose and why? Thanks

 

Oil pressure with a warning light.

 

Oil pressure. The car will still run with toasty oil, but it'll knock a hole in the block pretty quickly with no oil pressure! Plus, you can somewhat infer oil temp from oil pressure (hot oil = lower pressure).

 

is it hard to put in an oil pressure gauge (aka much wiring?)