For a playthrough, Kilowattrel is probably the stronger choice. Let's have a look at what they each bring to the table to a team as an Electric type.
Kilowattrel
+ Slightly higher Special Attack stat of 105 (Bellibolt has base 103)
+ Brings a Flying type with it, which makes it useful against Gym Leaders Katy and Brassius, as well as Eri from Team Star, alongside the obvious Gym Leader Kofu and Elite Four Larry battles
+ Is much, much faster than Bellibolt
+ Great ability choices in Wind Power and Volt Absorb
(-) Is much frailer than Bellibolt
(-) Gets additional weaknesses due to the Flying typing
Bellibolt
+ Much bulkier than Kilowattrel, useful against opponents with coverage
+ Although it has a slightly lower Special Attack stat, it's strongest hit usually hits harder than Kilowattrel thanks to Electromorphosis
+ Gets decent Water-type coverage to handle Ground types with Water Gun and later Water Pulse through TM, though these moves are not famed for their base power
(-) Much slower than Kilowattrel, meaning it almost always has to take a hit before attacking. This will cripple it when dealing with multiple Pokémon in battle
(-) Very short on coverage moves outside of a few Water-type moves and, very late in the game, Sucker Punch. This makes it useful in significantly less important battles.
So I think from that, the answer is pretty obvious. The only reason to use Bellibolt would be for the fact it can be evolved earlier, being right away with a Thunder Stone rather than waiting until level 25. But that alone is nowhere near enough to save Bellibolt.
Hope this helps!
