Have been to both Kotor and Herceg Novi and had a great time on each occasion, it will become the next Croatia in the next few years with the budget airlines establishing a hub at Tivat which gives direct access to the Bay of Kotor where the resorts are based.
Beautiful place and no shortage of amenities with plenty of places to eat and drink, facilities at the hotels we stayed at were both excellent, crossing the border from Dubrovnik can be a bit of a pain the longest delay we had was an hour, mind you we were there in May when it was a little bit quieter.
They have late summers so early September would be an excellent time to visit, obviously Dubrovnik is worth a visit if you've not been before, Kotor, Budvar and Herceg Novi all have pretty old towns, and Portonovi is where the footballers andk film stars stay.
We went into Bosnia for a day trip which was good and there are plenty of trips and things to see around Montenegro if you're that way inclined, prices are low, last year we were paying 15/20 euros for 3 courses plus wine and beer in nice restaurants.
Would certainly recommend and would definitely go again. Sorry I can't help with the resorts you've suggested as we didn't go but feel free to ask me any specifics that I might be able to help you with.
