Either option will work for you... it is not as hard as it seems up there. If you go the overland route I would recommend Africa Travel Co who run tours from Nairobi to Dar or vice versa which include the places you want to see, or if you want to fly to Arusha I would recommend Predators Safaris - local operator who do scheduled and tailor made packages that you can tune to suit yourself.
Don't forget though that part of the world is very easy to get around on public transport! Can get a bus to Arusha from Nairobi (took us 7 hours and apparently that was a bit slow!) and then organize a Serengeti trip from there. After that get on another bus to Dar (took us 14 hours). It just depends if you can be bothered sitting in a smelly bus for that long.
Some hotels in Nairobi and Arusha can put you in contact with a organized shuttle service (smaller and therefore quicker bus) that operates between Nairobi and Arusha as well. It was going to cost us only $15 more to do that than the local bus but unfortunately was full on the day we wanted.