Ultron should definitely be a man in a suit. If gives the performance/character a much more tangible feel, and is almost certainly cheaper
They can do it like the Iron Man suits, with CGI "fillings"
If they don't want to force the actor to go full-on C-3PO
Also IIRC the ILM guys were saying it was easier to work with the CGI "fillings", as it improves the actor's range of motion so that a fully CGI double wasn't often necessary, while keeping a large portion of the costume completed for them