Both of these schools are great and I would happily send my children to either. For those that have made the choice between the two, (St Leos is also a third possibility if you are Catholic), in the end it has been made on very slight differences.
There is one obvious physical difference - Devonport Primary is at the base of Mt Victoria, so on a sloping site, and Stanley Bay is on a flat site.
As you said the best thing is to visit both, the head teacher will show you around and you can have a look and a feel for whether one suits your child's personality more.
There is always an advantage to living close to your chosen school, not just for the short trip to school, but also for forming friendships where they can easily pop round each others houses. So you may also want to look at where you end up renting/buying? Generally those who live in Cheltenham go to Devonport Primary and those in Stanley Point go to Stanley Bay and then in central Devonport between the two schools it's probably 50/50.
Another way to get a feel for the schools is to check out their websites and read their weekly newsletter where they will talk about what activities are on offer.
Both schools offer sports although kids mainly do sports via the local football, rugby, cricket, tennis clubs etc.
I wouldn't say the parents are particularly chilled out in Devonport - it's an upper middle class area, with parents who are highly involved in the school system - and they all have an opinion!
However, that is the price you pay for schools that offer so much and do so well and it's a far cry from a uniformed prep school model.
Discipline is there but in my experience not really needed except in rare situations. Emphasis is more on self management and creating children who feel part of the school community.
I've always found Devonport to be very friendly and have rarely come across snobbery. It is, however, a transient place, people have arrived from all over NZ and the world and often head off again, which can mean friendship break ups. Many are obsessed with housing, (how much it costs and renovating villas), and rows in the local paper can go on forever over things like cycle lanes - very much a villagey feel in that respect.
As a location it is hard to beat for those who like, or need, to be close to Auckland. It always has something going on, even more so now the Victoria Theatre is being re-opened, it has easy access to the beach and lots of cafes. It is also very inclusive of children, not only with activities but how people talk to them.
It's probably not the definitive answer you want to hear but your sons would probably thrive at both schools.
Cheers
Tia