Depends what you're after. If it's acres and acres of empty space, peace, quiet, glorious sunsets etc. etc., then Portland is the place for you. It is also but a short walk from the fleshpots of Weymouth. However, you could go into Weymouth Harbour and raft up ten deep and have feet trampling all over your deck 24 hours a day and enjoy the thrill of picking up discarded fish and chip wrappers from your cockpit. I know where I'd rather be!
I bow to psygoddu's greater and more recent experience.
As far as I am aware, there are no visitors moorings in Portland and the fetch inside this massive area, can be considerable in a blow so you have to choose your anchorage with care. As an abandoned naval base the bottom in many parts is foul and there are restictions on both navigation and anchoring. Ashore facilities are few though the Castle Cove Sc and Weymouth Sailing Centre are reputed to be welcoming. The pilot books show an anchorage in Small Mouth, halfway round the western curve of the harbour with a note that holding here is good.
I take Pysgoddu's point about solitude. However, I would use words such as 'bleak' and 'abandoned'. I quite like Weymouth and have not suffered the assault of 'grockles', though I know others have.