I have both Future Sight and Psybeam and believe it or not but I prefer attacking gyms with my PB. It's just personal preferences but I like the 2 bar move with Confusion which is doing the job pretty well and charges the bars very quickly. I like to keep my 2nd bar for the next defender before switching my attacker.
Maybe some will disagree but it's just the way I prefer when it comes to attacking gyms with Espeon.