I dont think they do a Atlantic with a Diesel as in the main its only 6+mtrs long.Its designd for outboards and this vesell will cost a lot to convert and it isnt worth it nowadays.Its a great boat as is.
The pacific rib depends what size and engines your thinking of ?.
There were a few going cheap from one of the Disposal Companys that was advertised on here.
Both good boats but its all down to your use and realistick affordable running costs.
There is also the Arctic that I believe you cant beach as easy as the atlantic as it has a fuller deep V but rides a sea well.
There is a lot to your questions and you have a good boat at present, if you want to go bigger,and running costs are important then dependant on budget but a Humber Pro with a diesel/leg seem to me a good balanced buy.
This is my view and others will have there own views,so use it as a start.