If you only have 150GB left on a 500GB, without a doubt go for a 1TB; if you have 150GB left on a 256GB or 320GB, then a 500GB is likely all you need. With the way prices have dropped recently, namely with 1TB often dropping to the $170-190 price point on sale; it is hard to not upgrade that high. Luckily, your macbook is a generation where apple still allowed you to do upgrades.
Though you might not need it, iFixit has some great teardown info for the Mid-2012 MacBook Pro 13":
https://www.ifixit.com/Device/MacBoo...ibody_Mid_2012