It depends on the purpose of the phone. If it's just for making phone calls, then it doesn't really matter what you get. However if you, for example, have a lot of music that you want to be able to listen to through your phone, round up to the appropriate size (so that you will not have more room for music among other things).
If you want to have a lot of applications or movies on your phone, get a model with a higher storage space, i.e. 64GB (or if need be, 128GB, but I doubt that anyone would need this much space unless they work with multimedia). I personally have the 64GB version with 50GB free space. Keep in mind that a fair amount of space will be allocated to your iOS (around 2GB) and pictures/videos if you take any. Any videos recorded on your iPhone are not compressed very well at all, so they're quite large files... It is all up to your budget however, so depending on that find the right iPhone for you.