I didn't like Terraria after getting it for a dollar, and I felt the LFD2 demo was...lacking. However, they are good choices based on what the OP's requirements.

@daniel_nistor19:
There are ways to increase a console's firepower mid-gen, such as Nintendo's old expansion packs. However, it'd only fracture the user base and place players on an uneven footing. Cloud based boosting will help, but it'll fracture the user base nearly as much as having two console levels.

@Wickerman777:
Ugh. The power included in both consoles is just fine. 1080p/4k and 60 fps is less important than the price and experiences provided by the X1 and PS4.

Depending on the number of players, I advise the Borderlands series.

*sees troll bait*
*smells troll bait*
"No thanks. You can keep your rotten bait." :D

@Wickerman777:
If neither ecosystem is a big deal, then get the X1. It has several advantages, such as EA Access and more exclusives, versus the PS4s two advantages of a better GPU and more JRPGs. Regardless of what you choose, have fun! :)

Neither, since it depends on what they accomplished by not including splitscreen play. Also, there is plenty of Halo split screen, as well as other games, on the X1.

"Be careful not to overuse it though, because I know a lot of people don't consider it fair game when it comes to Shotgun battles, so see if your opponents use it first before doing so yourself."

Uh, no. I've never even heard of this technique, since I was a Horde and campaign player primarily, but anything is fair game in versus multiplayer. If something is that devastating, it needs something to balance it.

"NBA Live 16 September 29, 2015"

Taking time to the Vault bets now. Offering 2 to 1 for before November 1st 2015! :D

"Are you a Marcus, Dom, Cole, Baird, or Jack?"
Really? No love for the sisters of COG?