I prefer BnS over Tera but BnS has made some design decisions I'm unsure of if it's just a beta thing. I don't think one is inherently better than the other, just up to preference.
I've only played the NA CBT version of BnS (i.e. none of the other servers) so I think I might have different experiences from others.
The feel of the game itself is very similar, like the characters and such. BnS has a main storyline to go through whereas Tera I feel is less cohesive in a story sense, but the world exploration is a lot greater IMO.
There's no running or gliding or whatever in Tera, but there's also no mounts or pegasus in BnS. The pegasus flying lets you see a huge portion of the Blade & Soul Gold and introduces you to the area, but BnS relies more on the story to introduce you to the areas.
The character creation in BnS is much better than Tera, with many more options. The combat system is definitely different, especially the move chaining which I do find more intuitive than Tera.
Tera does have its collisions and objects working better. For instance, some arches that I feel like I should be able to walk under are just invisible walls. As well, there are a lot more loading screens in BnS than in Tera, with some places that serve nothing but for a transition. Possibly a CBT thing, I don't know.
Tera feels more like a F2P version of WoW, whereas BnS feels like it's trying to do something similar but in a new way. In the end, it's up to your preference.