I have much more fun with Brand due to his synergy with magic penetration along with
and
and he is just a walking nuke.
Xerath would be more Siegy than Brand and have more artillery so to speak due to his massive range.

Pros:
-Scales well with nearly every AP item if not every one
-Massive amounts of AoE and can decimate a team fight
-Even with just a little AP, you are still a walking nuclear warhead
-Magic penetration works wonders on him
-Somewhat of a lane bully depending on how you play him
-Doesn't have too much of a problem putting a dent in tanks
-Zombie Brand skin OP
-Easy to play if you get how his abilities work
Cons:
-No mobility
-Somewhat skill shot based (Q and W)
-Early mana issues
-No sustain
-Easy to play, hard to master
-Only thing he can peel / escape with is his stun, which is a skill shot and the target has to be afflicted with his passive first

Pros:
-One of the highest range champions in the game if not the highest
-Poke has extraordinarily long range
-Has great mana sustain due to his passive
-Can be strong even when a bit behind
-Can harass and be a major lane bully due to his Q and other abilities
-Siege potential
-Scales well into the game
Cons:
-No mobility
-Skill shot reliant (EVERYTHING is a skill shot)
-A bit hard to play, even harder to master (moreso than Brand)
-Q and W have a wait time (Q can be released whenever you want so not too bad)
-No escape, but has some form of peel with his W and E (more peel than brand)
-Has to go up close to get his passive off
-His ultimate can be hard to land
Both champs have great harass, wave clear, scaling, power to come back when behind, and still do well when behind.
Both champs suffer from low mobility, no escapes, a high skill cap, need to position, and having to land skill shots.