nebulous

the windows one is bedrock, right?
pretty sure its java thats the best one. at least thats the one i prefer.

are there any major differences between the two?

Java is way further with content while bedrock is still behind. Also bedrock its easier to play on vr while on Java you can but you have to install a mod. I would buy java if I was you. More content and more servers to play on

can i play on realms with friends for free

from what i can tell:
bedrock supports crossplay between multiple platforms, while java doesnt.
java has way more mods as its been around for a very long time
bedrock has controller support without the need of third party programs
i think theres also some differences regarding in-game functions like redstone, but im not entirely sure how much has changed.
im sure there are some site that show the differences

You get a realm free for a month when buying a account but after that you will have to pay.