[quote=@Laue]
Looking at Helltank's character John seems to be a bit inferior, except for the part where she's a little girl and John is a genius scientist. Then again, pretty much every mutation John possesses has a nasty surprise.
[/quote]

John has flexibility of attack and has a wide range of attack angles and types. Alison basically goes "hit hin, then shoot him, then hit him". Also, I believe John is more durable and the intelligence gap(homeschooled 11 year old vs genius) is a huge advantage.

I would say they are roughly equal, Alison's raw power and unpredictable nature may offer her an edge against monsters.