Farmland Tiers:

Base Soil: Mainly just an added crafting material, can still be used to grow crops faster than vanilla farmland.. (2x Speed)

Stone Soil: Quite useful when starting out, and not that expensive either.. (8x Speed)

Iron Soil: This is probably going to be the most used be people, very cheap to craft and decent speed.. (16x Speed)

Gold Soil: If you have the gold why not its quite fast.. (32x Speed)

Diamond Soil: Now this is what I'd use for most of my farms.. Very fast and useful.. (64x Speed)

Emerald Soil: You already crafted the diamond soil and need to do something with your stacks of emeralds? This is for you! (128x Speed)

Nether Star Soil: I don't recommend using this for efficiency as it is ridiculously expensive, but sometimes you have millions of nether stars and nothing to do with them.. (256x Speed, [Nearly Instant])

Farmland Features:

Tile Entity: Non-existent.. The soil uses random tick updates to grow crops and has no need for tiles.. Have fun placing hundreds without worrying if the soil will lag-up your world..

Hydrated: Why would you craft these expensive soils, if you needed water for it.. (Doesn't require water sources)

Indestructible: By this I mean nobody can trample your crops.. Feel free to jump around your farms..

Better Farmland