I swear by Dr Brown's bottles! We too had leaking issues but, as Hunnie said, it's usually when boiling water is used. I actually found that they leaked the most when, after the water had been in the fridge, I was reheating them in a jug of hot water. Not sure why.
My LO takes them at room temp now so we don't have the leaking problem anymore. When transporting them in a changing bag it's best to buy the caps/lids that go with them - they are a lot stronger.
Over here in Australia (not sure if different over there), the teats range from Level 1 to Level 3. My LO is almost 6 months so we've just started the Level 3.
I have used both the Avent steriliser and the Dr Brown's one and they are both great in terms of size for these bottles...oh yeah, we use the 'wide neck' ones too. I haven't tried the 'Standard neck'
Good luck with them!