An air tight seal on a dosing container with a peristaltic pump would mean you are drawing a vacuum into it, which is probably not good long term (and may not work anyway). A connection at the top then doesn't need to seal anything as you probably are letting air in anyway, and at the top it can hardly leak liquids...
A fitting at the top looks prettier, and may be handy for maintenance (if its a push fitting especially), but that is up to you.
I use BRS jugs with the 'mixing cap' with a hole drilled into it for my dosing containers. The seal is 'splash proof' at best but leaks enough air so there is never a vacuum in them.