Well, that depends on what you're looking for mostly.
Seaking is more physical and has a bit more bulk and can take advantage of lightning rod to nullify one of its weaknesses, while Lumineon is more offensively oriented/a bit faster and gets access to Storm Drain, making it a good switch in against water attacks (and due to it being a special attacker, makes more use of the sp. atk. boost than Seaking does).
In terms of offensive movepools, Seaking gets Drill Run, Megahorn, Waterfall, and Poison Jab for good physical coverage while Lumineon has access to Ice Beam, Scald, Signal Beam/Silver Wind, and Psybeam.
Personally, I like Lumineon a lot more due to bias/experiences using it, but overall Seaking is probably the better option, though the choice will ultimately be up to you for which one you wanna use.