Is there a difference between one being P2P and one being BT? Why not merge them together? Can anyone explain this? It feels like these two software are very similar.I'm confused about what to choose.

They are different software. One uses the BITTORRENT network and the other uses the FOPNU network.

Torrent clients like Tixati are meant to share content with all the world. Fopnu in theory can do that as well, but in my opinion it's more suitable for sharing files between friends, colleagues, and family members.  
I think The 2 programs are not merged together just to keep them lightweight.